feat(translation): 8h sweep + drain button + detection probe (#1376)

Throughput: translate_posts now runs every 8h (was daily) as the
steady-state cadence for newly-imported posts, and the Settings
"Translate now" button runs it in drain mode (run-until-done, no reset)
so one press clears the whole untranslated backlog instead of a single
300-post chunk. The interrupt/backoff re-enqueue now preserves the drain
flag so a bulk drain resumes cleanly after an Interpreter restart.

Misdetection groundwork: surface the detector's confidence from the
Interpreter client (it was in the detectedLanguage payload but discarded)
and add a read-only "Test translation" box — POST /settings/translation/
probe + TranslationCard UI — that shows detected language + confidence +
engine + result for pasted text, without saving. Lets the operator see
why a short/abbreviation-heavy English title gets mis-detected so the
detection guard (min-length + confidence floor) can be tuned from real
numbers. The guard itself follows once the mis-detected cases are probed.

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

backend/app/celery_app.py

@@ -178,9 +178,13 @@ def make_celery() -> Celery:
                 "task": "backend.app.tasks.ml.prune_presentation_reviews",
                 "schedule": 86400.0,  # retention: drop resolved review flags >30d
             },
-            "translate-posts-daily": {
+            "translate-posts-8h": {
                 "task": "backend.app.tasks.translation.translate_posts",
-                "schedule": 86400.0,  # no-op unless translation configured + healthy
+                "schedule": 28800.0,  # every 8h: steady-state cadence for the
+                # trickle of newly-imported posts (no-op unless translation
+                # configured + healthy). One bounded 300-chunk per fire — the
+                # one-time backlog drains via the "Translate now" button
+                # (drain=True, run-until-done), not this sweep.
             },
             "snapshot-head-metrics-daily": {
                 "task": "backend.app.tasks.maintenance.snapshot_head_metrics",