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/import_task.py

@@ -13,10 +13,12 @@ from sqlalchemy import (
     Boolean,
     DateTime,
     ForeignKey,
+    Index,
     Integer,
     String,
     Text,
     func,
+    text,
 )
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
@@ -26,6 +28,10 @@ from .base import Base
 class ImportTask(Base):
     __tablename__ = "import_task"
 
+
+    __table_args__ = (
+        Index("ix_import_task_created_at_desc", text("created_at DESC")),
+    )
     id: Mapped[int] = mapped_column(Integer, primary_key=True)
     batch_id: Mapped[int] = mapped_column(
         ForeignKey("import_batch.id", ondelete="CASCADE"), nullable=False, index=True
@@ -33,14 +39,14 @@ class ImportTask(Base):
 
     source_path: Mapped[str] = mapped_column(Text, nullable=False)
     task_type: Mapped[str] = mapped_column(String(16), nullable=False)  # media|archive
-    status: Mapped[str] = mapped_column(String(16), nullable=False, default="pending", index=True)
+    status: Mapped[str] = mapped_column(String(16), nullable=False, default="pending", index=True, server_default="pending")
 
     # Poison-pill circuit breaker (alembic 0026). recovery_count tracks
     # how many times the stuck-task sweep has re-queued this row; after
     # the cap it's failed with a diagnostic instead of looping. refetched
     # bounds the one-shot re-download remediation to a single attempt.
-    recovery_count: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
-    refetched: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
+    recovery_count: Mapped[int] = mapped_column(Integer, nullable=False, default=0, server_default="0")
+    refetched: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False, server_default="false")
 
     result_image_id: Mapped[int | None] = mapped_column(
         ForeignKey("image_record.id", ondelete="SET NULL"), nullable=True