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/source.py

@@ -5,7 +5,7 @@ Multiple sources per artist support creators with cross-platform presence.
 
 from datetime import datetime
 
-from sqlalchemy import JSON, Boolean, DateTime, ForeignKey, Integer, String, Text
+from sqlalchemy import Boolean, DateTime, ForeignKey, Integer, JSON, String, Text
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from .base import Base
@@ -20,7 +20,7 @@ class Source(Base):
     )
     platform: Mapped[str] = mapped_column(String(64), nullable=False)
     url: Mapped[str] = mapped_column(Text, nullable=False)
-    enabled: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
+    enabled: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True, server_default="true")
 
     config_overrides: Mapped[dict | None] = mapped_column(JSON, nullable=True)
 
@@ -32,7 +32,7 @@ class Source(Base):
     # by _update_source_health alongside last_error; cleared on 'ok'.
     error_type: Mapped[str | None] = mapped_column(String(32), nullable=True, index=True)
     check_interval_override: Mapped[int | None] = mapped_column(Integer, nullable=True)
-    consecutive_failures: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    consecutive_failures: Mapped[int] = mapped_column(Integer, nullable=False, default=0, server_default="0")
 
     # alembic 0031: sticky deep-scan budget. When > 0, the next N download
     # runs use gallery-dl's full-walk config (skip: True + 1800s timeout);