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_settings.py

@@ -17,60 +17,71 @@ class ImportSettings(Base):
     __table_args__ = (CheckConstraint("id = 1", name="singleton"),)
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True)
-    import_scan_path: Mapped[str] = mapped_column(Text, nullable=False, default="/import")
+    import_scan_path: Mapped[str] = mapped_column(Text, nullable=False, default="/import", server_default="/import")
 
-    min_width: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
-    min_height: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    min_width: Mapped[int] = mapped_column(Integer, nullable=False, default=0, server_default="0")
+    min_height: Mapped[int] = mapped_column(Integer, nullable=False, default=0, server_default="0")
 
-    skip_transparent: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
-    transparency_threshold: Mapped[float] = mapped_column(Float, nullable=False, default=0.9)
+    skip_transparent: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False, server_default="false")
+    transparency_threshold: Mapped[float] = mapped_column(Float, nullable=False, default=0.9, server_default="0.9")
 
-    skip_single_color: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
-    single_color_threshold: Mapped[float] = mapped_column(Float, nullable=False, default=0.95)
-    single_color_tolerance: Mapped[int] = mapped_column(Integer, nullable=False, default=30)
+    skip_single_color: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False, server_default="false")
+    single_color_threshold: Mapped[float] = mapped_column(Float, nullable=False, default=0.95, server_default="0.95")
+    single_color_tolerance: Mapped[int] = mapped_column(Integer, nullable=False, default=30, server_default="30")
 
-    phash_threshold: Mapped[int] = mapped_column(Integer, nullable=False, default=10)
+    phash_threshold: Mapped[int] = mapped_column(Integer, nullable=False, default=10, server_default="10")
 
     # FC-3c downloader knobs
     download_rate_limit_seconds: Mapped[float] = mapped_column(
-        Float, nullable=False, default=3.0
+        Float, nullable=False, default=3.0,
+        server_default="3",
     )
     download_validate_files: Mapped[bool] = mapped_column(
-        Boolean, nullable=False, default=True
+        Boolean, nullable=False, default=True,
+        server_default="true",
     )
 
     # FC-3d scheduling knobs
     download_schedule_default_seconds: Mapped[int] = mapped_column(
-        Integer, nullable=False, default=28800
+        Integer, nullable=False, default=28800,
+        server_default="28800",
     )
     download_event_retention_days: Mapped[int] = mapped_column(
-        Integer, nullable=False, default=90
+        Integer, nullable=False, default=90,
+        server_default="90",
     )
     download_failure_warning_threshold: Mapped[int] = mapped_column(
-        Integer, nullable=False, default=5
+        Integer, nullable=False, default=5,
+        server_default="5",
     )
 
     # FC-3h backup knobs.
     backup_db_nightly_enabled: Mapped[bool] = mapped_column(
         Boolean, nullable=False, default=False,
+        server_default="false",
     )
     backup_db_nightly_hour_utc: Mapped[int] = mapped_column(
         Integer, nullable=False, default=3,
+        server_default="3",
     )
     backup_db_keep_last_n: Mapped[int] = mapped_column(
         Integer, nullable=False, default=14,
+        server_default="14",
     )
     backup_images_keep_last_n: Mapped[int] = mapped_column(
         Integer, nullable=False, default=3,
+        server_default="3",
     )
 
     # FC-6.3 series continuation matcher. enabled gates the rescan; threshold is
     # the weighted-score cut-off (0..1) above which a pending suggestion is made.
     series_suggest_enabled: Mapped[bool] = mapped_column(
         Boolean, nullable=False, default=True,
+        server_default="true",
     )
     series_suggest_threshold: Mapped[float] = mapped_column(
         Float, nullable=False, default=0.5,
+        server_default="0.5",
     )
 
     # #830 off-platform file-host downloads — per-host enable lever (default on,