feat(translation): backfill sweep + beat + manual trigger (#143 step 3)

tasks/translation.py — translate_posts: picks untranslated posts (title OR
description non-empty), per-post [title, description] batch via the Interpreter
client, stores translations + detected lang + engine_version; passthrough /
already-target posts are marked handled with no stored translation. 503 or a
connection error interrupts (retry next cycle), 400 stops (fix config), per-post
commit keeps progress; wall-clock bounded. Wired into celery (maintenance_long
lane) + a daily beat. No-op unless enabled + base URL set + healthy. GET
/settings/translation/status + POST .../run for the Settings card. Task tests
(stubbed client, monkeypatched session).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/api/settings.py

@@ -1,12 +1,23 @@
 """Settings API: import filters, system stats."""
 
+import asyncio
 import secrets
 
 from quart import Blueprint, jsonify, request
-from sqlalchemy import func, select
+from sqlalchemy import func, or_, select
 
 from ..extensions import get_session
-from ..models import AppSetting, Artist, ImageRecord, ImportBatch, ImportSettings, ImportTask, Tag
+from ..models import (
+    AppSetting,
+    Artist,
+    ImageRecord,
+    ImportBatch,
+    ImportSettings,
+    ImportTask,
+    Post,
+    Tag,
+)
+from ..services import interpreter_client as ic
 
 settings_bp = Blueprint("settings", __name__, url_prefix="/api")
 
@@ -285,3 +296,45 @@ async def rotate_extension_api_key():
             row.value = new_value
         await session.commit()
     return jsonify({"key": new_value})
+
+
+# --- Translation (#143): live status + manual "Translate now" --------------
+
+
+@settings_bp.route("/settings/translation/status", methods=["GET"])
+async def translation_status():
+    """For the Settings card: is it on, is a URL set, is the service reachable,
+    and how many posts still await translation. Health runs the sync client in a
+    thread so the event loop isn't blocked."""
+    async with get_session() as session:
+        cfg = await ImportSettings.load(session)
+        untranslated = (await session.execute(
+            select(func.count(Post.id))
+            .where(Post.translated_source_lang.is_(None))
+            .where(or_(
+                Post.post_title.is_not(None), Post.description.is_not(None),
+            ))
+        )).scalar_one()
+    base_url = (cfg.interpreter_base_url or "").strip()
+    healthy = await asyncio.to_thread(ic.health, base_url) if base_url else False
+    return jsonify({
+        "enabled": cfg.translation_enabled,
+        "base_url_set": bool(base_url),
+        "healthy": healthy,
+        "untranslated_count": int(untranslated),
+    })
+
+
+@settings_bp.route("/settings/translation/run", methods=["POST"])
+async def translation_run():
+    """Enqueue the translate sweep now (the Settings 'Translate now' button)."""
+    async with get_session() as session:
+        cfg = await ImportSettings.load(session)
+    if not cfg.translation_enabled or not (cfg.interpreter_base_url or "").strip():
+        return jsonify(
+            {"error": "translation is disabled or no base URL is set"}
+        ), 400
+    from ..tasks.translation import translate_posts
+
+    r = translate_posts.delay()
+    return jsonify({"celery_task_id": r.id}), 202

backend/app/celery_app.py

@@ -35,6 +35,7 @@ def make_celery() -> Celery:
             "backend.app.tasks.backup",
             "backend.app.tasks.admin",
             "backend.app.tasks.library_audit",
+            "backend.app.tasks.translation",
         ],
     )
     app.conf.update(
@@ -63,6 +64,9 @@ def make_celery() -> Celery:
             "backend.app.tasks.backup.*": {"queue": "maintenance_long"},
             "backend.app.tasks.admin.*": {"queue": "maintenance_long"},
             "backend.app.tasks.library_audit.*": {"queue": "maintenance_long"},
+            # Translation backfill hits the LLM (~1–6s/item) → the long lane so it
+            # never starves the quick self-healing sweeps (#143).
+            "backend.app.tasks.translation.*": {"queue": "maintenance_long"},
         },
         # Heavy ML tasks need fair dispatch — see ImageRepo's precedent.
         task_acks_late=True,
@@ -161,6 +165,10 @@ def make_celery() -> Celery:
                 "task": "backend.app.tasks.ml.prune_presentation_reviews",
                 "schedule": 86400.0,  # retention: drop resolved review flags >30d
             },
+            "translate-posts-daily": {
+                "task": "backend.app.tasks.translation.translate_posts",
+                "schedule": 86400.0,  # no-op unless translation configured + healthy
+            },
             "snapshot-head-metrics-daily": {
                 "task": "backend.app.tasks.maintenance.snapshot_head_metrics",
                 "schedule": 86400.0,

backend/app/tasks/translation.py

@@ -0,0 +1,101 @@
+"""Post-text translation Celery task (milestone 143).
+
+Backfills non-English post title/description to the target language via the
+self-hosted Interpreter LAN service, storing the result + engine_version so
+viewing is instant and re-runs are cache-fast. Sync (Celery workers are sync),
+same pattern as the other backfill sweeps. No-op unless translation is enabled,
+a base URL is set, and the service is healthy.
+"""
+import logging
+from datetime import UTC, datetime
+
+from celery.exceptions import SoftTimeLimitExceeded
+from sqlalchemy import or_, select
+
+from ..celery_app import celery
+from ..models import ImportSettings, Post
+from ..services import interpreter_client as ic
+from ..utils.text import html_to_plain
+from ._sync_engine import sync_session_factory as _sync_session_factory
+
+log = logging.getLogger(__name__)
+
+# Bound one run's work so it commits progress rather than dying at the time
+# limit; the rest resumes next cycle (idempotent — only untranslated posts are
+# picked, so an interrupted run just re-runs = rule-89 recovery).
+_MAX_POSTS_PER_RUN = 300
+
+
+@celery.task(
+    name="backend.app.tasks.translation.translate_posts",
+    soft_time_limit=1800, time_limit=2100,
+)
+def translate_posts() -> str:
+    """Translate untranslated non-English post title/description (default target
+    en) via Interpreter. Per-post [title, description] batch → per-post detected
+    language. Passthrough / already-target posts are marked handled (source ==
+    target) with no stored translation. 503 or a connection error interrupts the
+    run (retry next cycle); 400 stops it (fix config); posts done so far stay
+    committed. No-op unless configured + healthy. Returns a summary string."""
+    SessionLocal = _sync_session_factory()
+    with SessionLocal() as session:
+        cfg = ImportSettings.load_sync(session)
+        if not cfg.translation_enabled or not cfg.interpreter_base_url.strip():
+            return "disabled"
+        base_url = cfg.interpreter_base_url.strip()
+        target = (cfg.translation_target_lang or "en").strip() or "en"
+        if not ic.health(base_url):
+            return "interpreter unavailable"
+
+        posts = session.execute(
+            select(Post)
+            .where(Post.translated_source_lang.is_(None))
+            .where(or_(
+                Post.post_title.is_not(None), Post.description.is_not(None),
+            ))
+            .limit(_MAX_POSTS_PER_RUN)
+        ).scalars().all()
+
+        translated = 0
+        try:
+            for post in posts:
+                translated += _translate_one(session, post, base_url, target)
+                session.commit()   # per-post → an interrupted run keeps progress
+        except ic.InterpreterUnavailable:
+            session.rollback()
+            return f"interrupted (service down) — translated={translated}"
+        except ic.InterpreterBadRequest as e:
+            session.rollback()
+            log.warning("translate_posts bad request: %s", e)
+            return f"stopped (bad request) — translated={translated}"
+        except SoftTimeLimitExceeded:
+            return f"time limit — translated={translated}"
+    return f"translated={translated} scanned={len(posts)}"
+
+
+def _translate_one(session, post, base_url: str, target: str) -> int:
+    """Translate one post's title/description in place. Returns 1 if it stored a
+    translation, 0 for passthrough/empty (which still marks the post handled)."""
+    title = (post.post_title or "").strip()
+    desc = (html_to_plain(post.description) if post.description else "") or ""
+    desc = desc.strip()
+    fields: list[tuple[str, str]] = []
+    if title:
+        fields.append(("title", title))
+    if desc:
+        fields.append(("desc", desc))
+    if not fields:
+        post.translated_source_lang = target   # nothing to translate → handled
+        return 0
+    res = ic.translate([t for _, t in fields], base_url=base_url, target=target)
+    detected = res["detected_lang"] or target
+    if detected == target or res["engine"] == "none":
+        post.translated_source_lang = target   # already target language → handled
+        return 0
+    by_field = {f: res["translations"][i] for i, (f, _) in enumerate(fields)}
+    post.post_title_translated = by_field.get("title")
+    post.description_translated = by_field.get("desc")
+    post.translated_source_lang = detected
+    post.translation_engine_version = res["engine_version"]
+    post.translated_at = datetime.now(UTC)
+    return 1