db: rename the four double-prefixed CHECK constraints (#3275)

Run 5026 got the models-vs-chain diff to 7 lines. Three findings, and one
of them reverses an assumption I made in the previous commit.

The doubled CHECK names are what the DATABASE has, not what the generator
invented. base.py's convention is ck_%(table_name)s_%(constraint_name)s,
which — unlike uq/fk/ix — applies even to a constraint that already has a
name, so four migrations that passed an already-prefixed name got it
prefixed twice:

    ck_import_settings_ck_import_settings_singleton
    ck_ml_settings_ck_ml_settings_singleton
    ck_post_ck_post_translation_override
    ck_tag_ck_tag_fandom_requires_character

The workflow repair added last commit is still correct and still needed —
autogenerate really does re-double a name on the round trip — but it was
making the MODELS side clean against a chain that is dirty. The
comment in ml_settings.py claiming its bare name "matches migration 0003"
was simply false; 0003 produces the doubled form.

Nothing reads a CHECK constraint by name, so this has never done harm.
But it is precisely the development-era residue the collapsed baseline
exists to leave behind, and a public schema should not ship it — so 0088
renames the deployed constraints and all six models now declare bare
names. RENAME CONSTRAINT is catalog-only: no scan, no rewrite, no
revalidation, which is why this is safe on post and tag. Guarded on
pg_constraint scoped by conrelid, so it is a no-op on a database built
from the models.

ix_tag_fandom_id showed as a difference only because chain_ref was pinned
to 0a5bbe8, which predates 0088 — the comparison was measuring the models
against a chain missing the migration that closes the gap. chain_ref now
defaults to blank, meaning "the chain in this ref". Pin it to a commit
only after the collapse, when the tree no longer carries the revisions.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017QHszn9H8VBvx5Ke8x1hvw

backend/app/models/post.py

@@ -41,7 +41,11 @@ class Post(Base):
         UniqueConstraint("source_id", "external_post_id", name="uq_post_source_external_id"),
         CheckConstraint(
             "translation_override IN ('auto', 'force', 'original')",
-            name="ck_post_translation_override",
+            # Bare name: Base.metadata's naming convention prepends
+            # ck_<table>_. Pre-prefixing it here doubles the prefix — see
+            # alembic 0088, which renames the four constraints that shipped
+            # that way (#3275).
+            name="translation_override",
         ),
     )