feat(translation): tunable acceptance floor (0.90) + per-post sticky override (#155)

The gate at a fixed 0.80 couldn't catch the real pain: Interpreter (fresh ==
cached, verified by probe) confidently mis-detects short ASCII English like
"... WIP Part 1" as German at 0.86 — above the floor — so it was accepted and a
re-translate reproduced it. Confidence alone can't separate the 0.86 collision
(genuine German lands there too), and single-word mis-flags sit at a confident
1.0 no floor catches.

Two operator-approved levers:

- Acceptance floor is now a live Settings value (ImportSettings.
  translation_min_confidence, default 0.90; surfaced in the Translation card), so
  it's tunable without a redeploy. _accept takes the threshold as a parameter.

- Per-post sticky override (Post.translation_override: auto/force/original).
  'force' stores a translation even below the floor (rescue a skipped
  legit-foreign title); 'original' keeps the original and clears any stored
  translation (kill a confident mis-flag no floor catches). The sweep honors it
  on every run and _reset_translations skips 'original', so the choice survives a
  Re-translate-all. POST /api/posts/<id>/translation-override applies it
  immediately (translate now when the service is up, else queue for the sweep).
  UI: PostTranslationControl on the posts-feed card.

Migration 0084 (both columns + a CHECK on the override). The feed + provenance
serializers expose translation_override.

With a stricter floor the rollback finally works: raise it -> Re-translate all ->
the 0.86 mis-flags are rejected and restored to the original; force /
keep-original handle the residual either way.

Tests: gate thresholds against the param (0.86 rejected at 0.90, explicit-floor
cases); sweep force/original + re-translate-skips-original; override endpoint
(validation, original clears, force queues when disabled, feed exposes it);
settings min_confidence default/save/validate.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CgZP9v2otxVJymiYsnVuMy

backend/app/models/import_settings.py

@@ -106,6 +106,15 @@ class ImportSettings(Base):
     translation_target_lang: Mapped[str] = mapped_column(
         Text, nullable=False, default="en", server_default="en",
     )
+    # The latin-script acceptance floor for the translation gate: a translation
+    # whose Interpreter-reported confidence is below this is kept as the original
+    # (operator-tunable, milestone 155). Default 0.9 — stricter than the old
+    # hardcoded 0.8, because Interpreter confidently mis-detects short ASCII
+    # English (e.g. "… WIP Part 1") as a European language at ~0.86. CJK stays
+    # trusted regardless (script-detected). Per-post overrides handle the misses.
+    translation_min_confidence: Mapped[float] = mapped_column(
+        Float, nullable=False, default=0.9, server_default="0.9",
+    )
 
     @classmethod
     async def load(cls, session) -> ImportSettings: