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/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,