db: collapse alembic 0001..0087 into one baseline (milestone 328 step 1)

87 revisions narrating this project's build-out become one file that
creates the schema in a single step. They cost nothing at runtime — all
86 upgrade steps ran in 0.2s (note #3260) — so this is a presentation
change, not a performance one: a new installer should not inherit our
development history to stand up a database.

Deleted: 87 revisions (6,052 lines), the 10 tests/test_migration_*.py
files (483 lines) that asserted intermediate states and backfills which
no longer exist, and backend/app/utils/artist_backfill.py — the only
live module a migration imported, with no other consumer anywhere. That
last one satisfies the operator's separate request to inline it into
0008 and delete the module; the squash removes both outright.

THE REVISION ID IS "0087", NOT "0001", ON PURPOSE. It is the id of the
last revision collapsed, so an existing database is already at head and
`alembic upgrade head` does nothing. The alternative is `alembic stamp`
against live data, and stamp validates NOTHING — it writes a version
string whether or not the schema matches, so a wrong baseline surfaces
later, via the next real migration, with no clean way back. This removes
that operation rather than making it safe. Future revisions run from
0088.

Four things are hand-written because SQLAlchemy metadata does not carry
them, and none fail at generation time:

  1. CREATE EXTENSION vector          — the VECTOR columns cannot be
     created without it, so it is ordered first in upgrade().
  2. CREATE EXTENSION tsm_system_rows — surfaces only when the random
     sample query runs.
  3. the HNSW index on image_record.siglip_embedding, raw SQL because
     create_index cannot express USING hnsw (... vector_cosine_ops).
     The quietest of the four: everything works, similarity search just
     stops using an index.
  4. import pgvector.sqlalchemy.vector — autogenerate EMITS
     pgvector.sqlalchemy.vector.VECTOR references without importing it,
     so the generated file dies with NameError on first run.

The candidate came out of CI (run 4967) as checksummed base64 rather
than a plain cat, because run 4964's cat was truncated mid-line inside a
column definition with the step still green — 29 tables instead of 42,
and it looked entirely plausible. Verified here: 56,582 bytes,
sha256 471acfca69c0…, 42 tables, 66 indexes, 42 drops.

NOT YET PROVEN against the old chain. baseline.yml does that, and it is
step 2's gate; this commit does not claim the schemas match.

alembic/versions/0022_source_per_artist_platform.py

@@ -1,223 +0,0 @@
-"""source-collapse: one Source per (artist, platform) — consolidate junk per-post Sources
-
-Revision ID: 0022
-Revises: 0021
-Create Date: 2026-05-26
-
-Closes the operator-flagged 2026-05-26 issue where the filesystem importer
-called _find_or_create_source(url=sd.post_url), creating one Source row per
-imported post URL. Operator's Atole artist had 406 Source rows where there
-should have been 1 (the /cw/Atole subscription Source).
-
-Source represents a subscription feed (one per artist+platform — the
-gallery-dl URL polled by the FC-3 downloader). Posts hang off it. The
-filesystem importer was misusing Source as a per-post key.
-
-Migration steps per (artist_id, platform) group with >1 Source:
-  1. Pick canonical — prefer a URL NOT matching '/posts/<id>$' (real
-     campaign URL like /cw/Atole); else min(id).
-  2. PRE-merge any Posts under non-canonical sources whose
-     external_post_id ALREADY exists under the canonical source. (Same
-     gallery-dl post imported via two different sidecar paths can plant
-     two Post rows with identical external_post_id under different
-     Sources for the same artist.) Repoint ImageProvenance +
-     ImageRecord.primary_post_id to the canonical-side Post, dedupe
-     ImageProvenance against alembic 0021's uq, then delete the
-     non-canonical-side Post. This MUST happen before step 3 — Postgres
-     fires uq_post_source_external_id row-by-row during the bulk UPDATE
-     and the merge-after-reparent ordering 500s on first collision
-     (operator-hit during v26.05.26.1 deploy, 2026-05-26).
-  3. Reparent remaining Posts onto canonical (no collisions possible now).
-  4. Reparent ImageProvenance.source_id off the non-canonical sources.
-  5. Delete the orphan Source rows.
-  6. If the canonical Source's URL still looks like a per-post URL (no
-     campaign URL existed among candidates), rewrite it to
-     'sidecar:<platform>:<artist_slug>' so the artist detail page shows
-     something readable.
-"""
-from typing import Sequence, Union
-
-from alembic import op
-from sqlalchemy import text
-
-revision: str = "0022"
-down_revision: Union[str, None] = "0021"
-branch_labels: Union[str, Sequence[str], None] = None
-depends_on: Union[str, Sequence[str], None] = None
-
-_POST_URL_RE = r"/posts/[^/]+$"
-
-
-def upgrade() -> None:
-    conn = op.get_bind()
-
-    # Find (artist_id, platform) groups with > 1 Source row.
-    groups = conn.execute(text("""
-        SELECT artist_id, platform
-        FROM source
-        GROUP BY artist_id, platform
-        HAVING COUNT(*) > 1
-    """)).fetchall()
-
-    for artist_id, platform in groups:
-        rows = conn.execute(
-            text("""
-                SELECT id, url FROM source
-                WHERE artist_id = :a AND platform = :p
-                ORDER BY id ASC
-            """),
-            {"a": artist_id, "p": platform},
-        ).fetchall()
-
-        # Canonical: first row whose URL doesn't look like a per-post URL;
-        # else min(id).
-        canonical_id = None
-        for sid, url in rows:
-            if not _matches_post_url(url):
-                canonical_id = sid
-                break
-        if canonical_id is None:
-            canonical_id = rows[0][0]
-
-        other_ids = [sid for sid, _ in rows if sid != canonical_id]
-        if not other_ids:
-            continue
-
-        # STEP 2: PRE-merge ALL Posts with duplicate external_post_id
-        # across the entire (canonical + others) group, BEFORE the bulk
-        # reparent. Two cases must both be handled:
-        #   (A) canonical has Post X with epid=N; an "other" source has
-        #       Post Y with epid=N → after bulk UPDATE, (canonical, N)
-        #       collides with itself.
-        #   (B) two different "other" sources each have a Post with
-        #       epid=N; canonical has none → after bulk UPDATE, both
-        #       are repointed to (canonical, N) and the second collides.
-        # The earlier version of this migration only handled (A); the
-        # operator's deploy 2026-05-26 tripped (B) at line 139.
-        # Fix: group ALL Posts in the (artist, platform) by epid; for
-        # any group with count>1, pick the keep (prefer one already
-        # under canonical; else lowest id) and merge the rest into it.
-        all_posts = conn.execute(
-            text("""
-                SELECT external_post_id, id, source_id
-                FROM post
-                WHERE source_id = :canonical OR source_id = ANY(:others)
-                ORDER BY external_post_id, id
-            """),
-            {"canonical": canonical_id, "others": other_ids},
-        ).fetchall()
-        by_epid: dict = {}
-        for epid, post_id, src_id in all_posts:
-            by_epid.setdefault(epid, []).append((post_id, src_id))
-        for _epid, posts in by_epid.items():
-            if len(posts) <= 1:
-                continue
-            # Prefer a Post already under canonical as the keep.
-            canonical_posts = [p for p in posts if p[1] == canonical_id]
-            if canonical_posts:
-                keep_id = canonical_posts[0][0]
-            else:
-                keep_id = posts[0][0]  # already sorted by id ASC
-            drop_ids = [p[0] for p in posts if p[0] != keep_id]
-            for drop_id in drop_ids:
-                # Pre-delete image_provenance rows under drop_ whose
-                # image_record_id ALREADY has a provenance under keep —
-                # the UPDATE below would otherwise repoint them and
-                # trip uq_image_provenance_image_post (alembic 0021)
-                # row-by-row before any after-the-fact dedupe could
-                # run. Operator's v26.05.26.3 deploy 2026-05-26 tripped
-                # this at line 123.
-                conn.execute(
-                    text("""
-                        DELETE FROM image_provenance
-                        WHERE post_id = :drop_
-                          AND image_record_id IN (
-                              SELECT image_record_id FROM image_provenance
-                              WHERE post_id = :keep
-                          )
-                    """),
-                    {"keep": keep_id, "drop_": drop_id},
-                )
-                # Now safe to repoint the survivors.
-                conn.execute(
-                    text("""
-                        UPDATE image_provenance SET post_id = :keep
-                        WHERE post_id = :drop_
-                    """),
-                    {"keep": keep_id, "drop_": drop_id},
-                )
-                conn.execute(
-                    text("""
-                        UPDATE image_record SET primary_post_id = :keep
-                        WHERE primary_post_id = :drop_
-                    """),
-                    {"keep": keep_id, "drop_": drop_id},
-                )
-                conn.execute(
-                    text("DELETE FROM post WHERE id = :drop_"),
-                    {"drop_": drop_id},
-                )
-
-        # STEP 3: Bulk reparent the remaining Posts off the other
-        # Sources. After step 2, no collisions on
-        # (canonical, external_post_id) are possible.
-        conn.execute(
-            text("""
-                UPDATE post SET source_id = :canonical
-                WHERE source_id = ANY(:others)
-            """),
-            {"canonical": canonical_id, "others": other_ids},
-        )
-
-        # STEP 4: Reparent ImageProvenance.source_id (denormalized FK).
-        # No UNIQUE on source_id; safe bulk update.
-        conn.execute(
-            text("""
-                UPDATE image_provenance SET source_id = :canonical
-                WHERE source_id = ANY(:others)
-            """),
-            {"canonical": canonical_id, "others": other_ids},
-        )
-
-        # STEP 5: Drop the orphan Sources.
-        conn.execute(
-            text("DELETE FROM source WHERE id = ANY(:others)"),
-            {"others": other_ids},
-        )
-
-        # If the canonical's URL still looks per-post (no campaign URL
-        # existed among the candidates), rewrite to a synthetic anchor so
-        # the artist detail page renders something readable.
-        canonical_url = conn.execute(
-            text("SELECT url FROM source WHERE id = :id"),
-            {"id": canonical_id},
-        ).scalar_one()
-        if _matches_post_url(canonical_url):
-            slug = conn.execute(
-                text("SELECT slug FROM artist WHERE id = :id"),
-                {"id": artist_id},
-            ).scalar_one()
-            conn.execute(
-                text("""
-                    UPDATE source
-                    SET url = :new_url, enabled = false
-                    WHERE id = :id
-                """),
-                {
-                    "id": canonical_id,
-                    "new_url": f"sidecar:{platform}:{slug}",
-                },
-            )
-
-
-def downgrade() -> None:
-    # Lossy migration — orphan Sources deleted, Posts reparented, Posts
-    # merged. No safe downgrade. If you need to roll back the schema
-    # invariant, fork from 0021 and re-run filesystem imports.
-    pass
-
-
-def _matches_post_url(url: str) -> bool:
-    """True if url ends with /posts/<token> (gallery-dl-style per-post URL)."""
-    import re
-    return bool(re.search(_POST_URL_RE, url or ""))