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/api/posts.py

@@ -3,12 +3,28 @@
 from quart import Blueprint, jsonify, request
 
 from ..extensions import get_session
+from ..models import ImportSettings, Post
+from ..services import interpreter_client as ic
 from ..services.post_feed_service import PostFeedService
 from ..services.source_service import KNOWN_PLATFORMS
+from ..utils.text import html_to_plain
 from ._responses import error_response as _bad
 
 posts_bp = Blueprint("posts", __name__, url_prefix="/api/posts")
 
+_TRANSLATION_OVERRIDES = ("auto", "force", "original")
+
+
+def _queue_for_sweep(post: Post) -> None:
+    """Mark a post untranslated (all translation columns NULL) so the periodic
+    sweep re-runs it under its new override — used when Interpreter is down and we
+    can't translate inline."""
+    post.post_title_translated = None
+    post.description_translated = None
+    post.translated_source_lang = None
+    post.translation_engine_version = None
+    post.translated_at = None
+
 
 @posts_bp.route("", methods=["GET"])
 async def list_posts():
@@ -82,3 +98,70 @@ async def get_post(post_id: int):
     if item is None:
         return _bad("not_found", status=404, detail=f"post id={post_id}")
     return jsonify(item)
+
+
+@posts_bp.route("/<int:post_id>/translation-override", methods=["POST"])
+async def set_translation_override(post_id: int):
+    """Sticky per-post translation override (milestone 155). Body:
+    ``{"override": "auto" | "force" | "original"}``.
+
+    'original' keeps the original (clears any stored translation now — no
+    Interpreter needed). 'force'/'auto' translate the post immediately if the
+    service is up (force bypasses the acceptance floor; auto re-runs the gate);
+    if it's down we save the flag and mark the post untranslated so the next sweep
+    applies it. The override persists, so the sweep + Re-translate-all keep
+    honoring it. Returns the updated translation fields + an ``applied`` status."""
+    body = await request.get_json(silent=True) or {}
+    override = body.get("override")
+    if override not in _TRANSLATION_OVERRIDES:
+        return _bad(
+            "invalid_override",
+            detail=f"override must be one of {list(_TRANSLATION_OVERRIDES)}",
+        )
+
+    # Lazy import (mirrors settings.py) so the API module doesn't pull the celery
+    # task graph at import time.
+    from ..tasks.translation import _store_translation, _translate_field
+
+    async with get_session() as session:
+        post = await session.get(Post, post_id)
+        if post is None:
+            return _bad("not_found", status=404, detail=f"post id={post_id}")
+        post.translation_override = override
+        cfg = await ImportSettings.load(session)
+        target = (cfg.translation_target_lang or "en").strip() or "en"
+
+        if override == "original":
+            _store_translation(post, (None, None, None), (None, None, None), target)
+            applied = "cleared"
+        else:
+            base_url = (cfg.interpreter_base_url or "").strip()
+            if cfg.translation_enabled and base_url and ic.health(base_url):
+                force = override == "force"
+                title = (post.post_title or "").strip()
+                desc = (html_to_plain(post.description) if post.description else "") or ""
+                desc = desc.strip()
+                mc = cfg.translation_min_confidence
+                try:
+                    title_res = _translate_field(title, base_url, target, mc, force=force)
+                    desc_res = _translate_field(desc, base_url, target, mc, force=force)
+                except ic.InterpreterUnavailable:
+                    _queue_for_sweep(post)
+                    applied = "queued"
+                else:
+                    _store_translation(post, title_res, desc_res, target)
+                    applied = "translated"
+            else:
+                # Disabled / no URL / unhealthy → let the sweep apply it later.
+                _queue_for_sweep(post)
+                applied = "queued"
+
+        await session.commit()
+        return jsonify({
+            "id": post.id,
+            "translation_override": post.translation_override,
+            "post_title_translated": post.post_title_translated,
+            "description_translated": post.description_translated,
+            "translated_source_lang": post.translated_source_lang,
+            "applied": applied,
+        })