feat(model): nullable Post.source_id + denormalized Post.artist_id; retire sidecar synthetics

Operator-asked 2026-06-01 after the Dymkens orphan investigation
(Scribe plan #540). The pre-2030 sidecar-synthetic Source pattern
(`sidecar:<platform>:<slug>` enabled=false rows) existed solely to
satisfy `Post.source_id NOT NULL`, and leaked into the Subscriptions
UI as phantom subscriptions. Now the data model says what's true:
filesystem-imported content with no live subscription has NULL
source_id, full stop.

## Schema (alembic 0030)

- `post.artist_id` — NEW NOT NULL FK to artist (CASCADE). Backfilled
  from source.artist_id in the migration. Indexed for the artist-filter
  queries.
- `post.source_id` — NOT NULL → nullable; FK ondelete CASCADE → SET
  NULL. Deleting a Source detaches its Posts instead of destroying
  archived content (subscription ends, archive stays).
- `image_provenance.source_id` — same nullable + SET NULL.
- Partial unique index `uq_post_artist_external_id_null_source` on
  (artist_id, external_post_id) WHERE source_id IS NULL — guards
  filesystem-import dedup since the existing source-bound unique
  ignores NULLs (Postgres treats NULL != NULL).
- Sidecar synthetic Sources deleted: NULL out FKs in post,
  image_provenance first, then DELETE FROM source WHERE url LIKE
  'sidecar:%'. The Dymkens cleanup.

## Model + service changes

- `Post.source_id` → `Mapped[int | None]`; new `Post.artist_id`
  denormalized.
- `ImageProvenance.source_id` → `Mapped[int | None]`.
- Importer: `_source_for_sidecar` (synthetic-creating) →
  `_lookup_source_for_sidecar` (returns None when no subscription).
  `_find_or_create_post` takes required `artist_id`; matches on
  (source_id, external_post_id) for source-bound posts or
  (artist_id, external_post_id) for NULL-source posts.
- Service queries switched off the Source detour to use Post.artist_id
  directly: post_feed_service.scroll/around/get_post (LEFT JOIN to
  Source so NULL-source posts surface); artist_service date_row/
  activity/post_count; provenance_service.for_image/for_post (LEFT
  JOIN); gallery_service._provenance_exists_where_artist via
  Post.artist_id instead of ImageProvenance.source_id → Source.
- `_to_dict` and provenance dict-builders emit `"source": null` for
  NULL-source rows.

## Frontend

- `ProvenancePanel.vue` + `PostCard.vue`: render `e.source?.platform
  ?? 'filesystem import'` so NULL-source posts get a clear
  "filesystem import" affordance instead of a NaN crash.

## Tests

- `test_importer_upsert_helpers`: removed the four synthetic-anchor
  tests; added `_find_or_create_post_idempotent_with_null_source`
  (dedup via the partial unique index) and
  `_lookup_source_for_sidecar_returns_*` (existing-subscription +
  none cases). The existing `_find_or_create_post_idempotent` now
  also passes `artist_id` and asserts it.
- 8 other test files updated: every direct `Post(...)` construction
  gains `artist_id=<artist>.id`. The `_seed_post` helper in
  `test_post_feed_service` looks up artist_id from the source row so
  callsites stay one-arg.

## Verification on deploy

After alembic 0030 runs:
- `SELECT COUNT(*) FROM source WHERE url LIKE 'sidecar:%'` → 0.
- `SELECT COUNT(*) FROM post WHERE source_id IS NULL` → count of
  filesystem-imported posts (Dymkens + any other historical).
- Every `post.artist_id` non-null; consistent with source.artist_id
  for source-bound rows.
- Subscriptions tab: no Dymkens phantom row.
- Artist detail → Posts/Gallery: Dymkens's content still reachable
  via Post.artist_id.
- Provenance panel renders "filesystem import" chip for NULL-source
  posts; PostCard same.

## Out of scope

- UI to manage/delete orphan NULL-source Posts. Data model is right;
  UI follows if operator wants it.

tests/test_importer_upsert_helpers.py

@@ -88,11 +88,36 @@ def test_find_or_create_post_idempotent(importer, artist_row, db_sync):
     )
     p1 = importer._find_or_create_post(
         source_id=src.id, external_post_id="ext-001",
+        artist_id=artist_row.id,
     )
     p2 = importer._find_or_create_post(
         source_id=src.id, external_post_id="ext-001",
+        artist_id=artist_row.id,
     )
     assert p1.id == p2.id
+    assert p1.artist_id == artist_row.id
+
+
+def test_find_or_create_post_idempotent_with_null_source(
+    importer, artist_row, db_sync,
+):
+    """Post.source_id is nullable since alembic 0030 — filesystem-imported
+    posts with no live subscription have NULL source_id. The partial
+    unique index `uq_post_artist_external_id_null_source` on
+    (artist_id, external_post_id) WHERE source_id IS NULL guards the
+    dedup; the helper matches on the same (artist_id, external_post_id)
+    keys when source_id is None."""
+    p1 = importer._find_or_create_post(
+        source_id=None, external_post_id="fs-001",
+        artist_id=artist_row.id,
+    )
+    p2 = importer._find_or_create_post(
+        source_id=None, external_post_id="fs-001",
+        artist_id=artist_row.id,
+    )
+    assert p1.id == p2.id
+    assert p1.source_id is None
+    assert p1.artist_id == artist_row.id
 
 
 def test_find_or_create_source_recovers_from_integrity_error(
@@ -147,13 +172,13 @@ def test_find_or_create_source_recovers_from_integrity_error(
     assert recovered.id == pre_existing.id
 
 
-def test_source_for_sidecar_reuses_existing_subscription(
+def test_lookup_source_for_sidecar_returns_existing_subscription(
     importer, artist_row, db_sync,
 ):
-    """The filesystem-import sidecar resolver should attach to whatever
-    Source already exists for (artist, platform) — the canonical subscription
-    Source — regardless of its URL. Without this, every imported post
-    spawned its own Source row.
+    """The sidecar-import Source lookup returns the existing Source for
+    (artist, platform) when one exists — letting filesystem-imported
+    content attach to the artist's real subscription instead of being
+    orphaned.
     """
     canonical = Source(
         artist_id=artist_row.id, platform="patreon",
@@ -162,84 +187,30 @@ def test_source_for_sidecar_reuses_existing_subscription(
     db_sync.add(canonical)
     db_sync.flush()
 
-    resolved = importer._source_for_sidecar(
+    resolved = importer._lookup_source_for_sidecar(
         artist_id=artist_row.id, platform="patreon",
-        artist_slug=artist_row.slug,
     )
+    assert resolved is not None
     assert resolved.id == canonical.id
 
 
-def test_source_for_sidecar_creates_synthetic_anchor_when_none_exists(
+def test_lookup_source_for_sidecar_returns_none_when_no_subscription(
     importer, artist_row, db_sync,
 ):
-    """No subscription Source for this (artist, platform) yet. The helper
-    creates one synthetic anchor (enabled=False, url='sidecar:<plat>:<slug>')
-    so subsequent imports reuse it instead of spawning per-post Sources.
+    """Alembic 0030 made Post.source_id nullable; the importer no
+    longer creates synthetic `sidecar:<platform>:<slug>` anchor rows
+    when no real subscription exists. The lookup returns None and the
+    caller carries that through as a NULL source_id on the new Post.
     """
-    resolved = importer._source_for_sidecar(
+    resolved = importer._lookup_source_for_sidecar(
         artist_id=artist_row.id, platform="pixiv",
-        artist_slug=artist_row.slug,
     )
-    assert resolved.url == f"sidecar:pixiv:{artist_row.slug}"
-    assert resolved.enabled is False
-    assert resolved.artist_id == artist_row.id
-    assert resolved.platform == "pixiv"
+    assert resolved is None
 
-    # Second call returns the same row (no new Source spawned).
-    again = importer._source_for_sidecar(
-        artist_id=artist_row.id, platform="pixiv",
-        artist_slug=artist_row.slug,
-    )
-    assert again.id == resolved.id
-
-
-def test_source_for_sidecar_distinct_platforms_distinct_anchors(
-    importer, artist_row, db_sync,
-):
-    """One synthetic anchor per (artist, platform). Different platforms get
-    different anchors even when no campaign Source exists for either.
-    """
-    p = importer._source_for_sidecar(
-        artist_id=artist_row.id, platform="patreon",
-        artist_slug=artist_row.slug,
-    )
-    x = importer._source_for_sidecar(
-        artist_id=artist_row.id, platform="pixiv",
-        artist_slug=artist_row.slug,
-    )
-    assert p.id != x.id
-    assert p.platform == "patreon"
-    assert x.platform == "pixiv"
-
-
-def test_source_for_sidecar_prefers_real_over_synthetic_when_both_exist(
-    importer, artist_row, db_sync,
-):
-    """When BOTH a synthetic anchor AND a real Source exist for the same
-    (artist, platform), the resolver must return the REAL one. This is the
-    fix for the 2026-05-31 phantom-subscription bug: alembic 0022 had
-    rewritten an older per-post Source row into a sidecar synthetic, and
-    the operator later added the real subscription. The old `ORDER BY
-    id ASC LIMIT 1` lookup picked the older synthetic (lower id),
-    silently attaching every gallery-dl download to the wrong Source.
-    """
-    synthetic = Source(
-        artist_id=artist_row.id, platform="patreon",
-        url=f"sidecar:patreon:{artist_row.slug}", enabled=False,
-    )
-    db_sync.add(synthetic)
-    db_sync.flush()
-    real = Source(
-        artist_id=artist_row.id, platform="patreon",
-        url="https://www.patreon.com/testartist", enabled=True,
-    )
-    db_sync.add(real)
-    db_sync.flush()
-    assert synthetic.id < real.id  # ordering precondition
-
-    resolved = importer._source_for_sidecar(
-        artist_id=artist_row.id, platform="patreon",
-        artist_slug=artist_row.slug,
-    )
-    assert resolved.id == real.id
-    assert resolved.url == "https://www.patreon.com/testartist"
+    # Verify no Source row was created as a side effect.
+    count = db_sync.execute(
+        select(Source).where(
+            Source.artist_id == artist_row.id, Source.platform == "pixiv",
+        )
+    ).all()
+    assert count == []