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.

backend/app/models/external_link.py

@@ -16,6 +16,7 @@ doesn't delete the link record).
 from datetime import datetime
 
 from sqlalchemy import (
+    CheckConstraint,
     DateTime,
     Float,
     ForeignKey,
@@ -38,6 +39,16 @@ STATUSES = ("pending", "downloading", "downloaded", "failed", "skipped", "dead")
 class ExternalLink(Base):
     __tablename__ = "external_link"
     __table_args__ = (
+        # alembic 0028 enum CHECKs. Rule 36 territory: a new host or status value
+        # needs its constraint swapped in the same migration (#3275).
+        CheckConstraint(
+            "host IN ('mega', 'gdrive', 'mediafire', 'dropbox', 'pixeldrain')",
+            name="ck_external_link_host",
+        ),
+        CheckConstraint(
+            "status IN ('pending', 'downloading', 'downloaded', 'failed', 'skipped', 'dead')",
+            name="ck_external_link_status",
+        ),
         # One row per (post, url). The full url (incl. #fragment) is the identity
         # — the same file linked twice in a post collapses to one row.
         Index("uq_external_link_post_url", "post_id", "url", unique=True),