db: reconcile the models with the deployed schema (#3275)

Milestone 328's acceptance test compared a database built by the real
0001..0087 chain against one built from the models, and found ~130
places where they disagree. This closes them.

Almost all were the MODEL being wrong, so almost all of this is model
edits with no DDL — the database already had these things, nothing in it
changes, and no deploy is needed for this part:

* 92 columns gained server_default. The models carried Python-side
  `default=` only, so the ORM filled the value and the column had no
  database default. Anything inserting outside the ORM behaved
  differently from production.

* Eleven indexes that existed only in migrations are now declared:
  the three backup_run reporting indexes, the two date-ordered
  image_record browse indexes, import_task and presentation_review,
  and the three task_run history indexes. All use text() for their DESC
  ordering and postgresql_where for the partial one.

* Two UNIQUE indexes that autogenerate silently proposed DROPPING,
  because neither is expressible as a UniqueConstraint:
    uq_tag_name_kind_fandom  — an EXPRESSION index over
                               (name, kind, COALESCE(fandom_id, 0))
    uq_post_artist_external_id_null_source — PARTIAL, WHERE source_id
                               IS NULL
  post.py already had a comment describing the second one. The comment
  was right; nothing declared it.

* The two external_link enum CHECKs (host, status) — rule 36 territory,
  and absent from the model entirely.

* Two indexes were named explicitly. A bare index=True generated
  ix_tag_alias_canonical_tag_id where the database has
  ix_tag_alias_canonical, so autogenerate proposed a drop+create of an
  index that was already there under another name. Same for
  tag_suggestion_rejection.

Only ONE thing needed DDL, as 0088: tag.fandom_id is declared
index=True but no migration ever created that index.

Deliberately NOT here: image_record.sha256. The model says unique=True;
0001 created a plain index. Duplicates are possible today and the ORM
believes otherwise. The fix depends on whether duplicates already exist
— if they do, that is a dedupe decision, not a constraint — so it waits
on an answer about live data.

The real severity of #3275 is not the squash. It is that --autogenerate
has been unsafe on this project: run against the old models it would
have proposed dropping eleven indexes and two uniqueness guarantees.

alembic/versions/0088_reconcile_models_with_schema.py

@@ -0,0 +1,48 @@
+"""Reconcile the database with what the models have always claimed (#3275).
+
+Milestone 328 discovered ~130 places where the ORM models and the deployed
+schema disagreed. Almost all of them were the MODEL being wrong — missing
+`server_default`s, indexes and CHECK constraints that only ever existed in a
+migration — and those are fixed in the model files with no DDL at all, because
+the database already had them.
+
+This migration carries the remainder: the one case where the MODEL was right
+and the database was missing something.
+
+`tag.fandom_id` is declared `index=True` on the model, but no migration ever
+created that index. Every autogenerate run since would have proposed adding
+it; nobody ran one, so the model and the database simply drifted apart and
+stayed that way.
+
+Deliberately NOT in this migration: making `image_record.sha256` unique. The
+model says `unique=True` and `0001` created a plain, non-unique index, so
+duplicates are possible today and the ORM believes they are not. Adding the
+constraint is a real change that FAILS if duplicates already exist, and if
+they do exist the right response is a dedupe decision rather than a constraint
+— so it needs an answer about live data before it is written, not after.
+Tracked in #3275.
+
+Revision ID: 0088
+Revises: 0087
+Create Date: 2026-08-30
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+revision: str = "0088"
+down_revision: Union[str, None] = "0087"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # IF NOT EXISTS because the index is what the model already asks for: any
+    # database built from metadata rather than from this chain will have it,
+    # and this migration must be a no-op there rather than an error.
+    op.execute("CREATE INDEX IF NOT EXISTS ix_tag_fandom_id ON tag (fandom_id)")
+
+
+def downgrade() -> None:
+    op.execute("DROP INDEX IF EXISTS ix_tag_fandom_id")