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

alembic/versions/0084_translation_strictness_override.py

@@ -0,0 +1,51 @@
+"""translation strictness setting + per-post translation override (milestone 155)
+
+ImportSettings gains ``translation_min_confidence`` (the latin-script acceptance
+floor, now operator-tunable in the UI; default 0.9 — stricter than the old
+hardcoded 0.8, since Interpreter confidently mis-detects short ASCII English at
+~0.86). Post gains ``translation_override`` — a sticky per-post choice of
+auto / force / original so the operator can force a skipped translation on, or
+knock a wrongly-translated one back to the original, and have it survive a
+Re-translate-all.
+
+Revision ID: 0084
+Revises: 0083
+Create Date: 2026-07-10
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0084"
+down_revision: Union[str, None] = "0083"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "import_settings",
+        sa.Column(
+            "translation_min_confidence", sa.Float(), nullable=False,
+            server_default=sa.text("0.9"),
+        ),
+    )
+    op.add_column(
+        "post",
+        sa.Column(
+            "translation_override", sa.String(16), nullable=False,
+            server_default="auto",
+        ),
+    )
+    op.create_check_constraint(
+        "ck_post_translation_override",
+        "post",
+        "translation_override IN ('auto', 'force', 'original')",
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint("ck_post_translation_override", "post", type_="check")
+    op.drop_column("post", "translation_override")
+    op.drop_column("import_settings", "translation_min_confidence")