feat(translation): pooled Interpreter session + manual sweep resumes after drain

- interpreter_client: shared requests.Session with a connect-only retry
  (connect=2, no status retries — we map 429/5xx ourselves) so a proxy reload
  is smoothed and the keep-alive connection is pooled across the sweep.
- translate_posts: on an interrupt (drain), re-enqueue after the Retry-After
  hint / default backoff instead of waiting for the daily beat; self-terminates
  via the health gate. Steady-state one-chunk-per-run on success is unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/tasks/translation.py

@@ -70,10 +70,21 @@ def translate_posts() -> str:
             return ready
         base_url, target = ready
         posts = _select_untranslated(session, None, _MAX_POSTS_PER_RUN)
-        # Steady-state daily sweep: one chunk per run. On interruption the
-        # untranslated rows stay NULL and the next daily beat (or a fresh
-        # "Translate now") resumes them — no self-chase needed here.
-        status, translated, _retry = _translate_batch(session, posts, base_url, target)
+        status, translated, retry_after = _translate_batch(
+            session, posts, base_url, target,
+        )
+        # Steady-state daily sweep: one chunk per run on success (the beat drives
+        # the rest — no tail-chase). But if Interpreter drained mid-chunk, don't
+        # make a manual "Translate now" wait a whole day — re-enqueue after its
+        # Retry-After hint (or the default backoff). Self-terminating: once the
+        # service is down the health gate returns "interpreter unavailable" early.
+        if status == "interrupted" and _count_untranslated(session, None):
+            delay = _interrupt_backoff(retry_after)
+            translate_posts.apply_async((), {}, countdown=delay)
+            log.info(
+                "translate_posts: interrupted (service draining) → retry in %ss",
+                delay,
+            )
     return _summary(status, translated, len(posts))