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/services/interpreter_client.py

@@ -12,6 +12,8 @@ from datetime import UTC, datetime
 from email.utils import parsedate_to_datetime
 
 import requests
+from requests.adapters import HTTPAdapter
+from urllib3.util.retry import Retry
 
 
 class InterpreterUnavailable(Exception):
@@ -57,13 +59,29 @@ def _parse_retry_after(resp) -> float | None:
     return max(0.0, (when - datetime.now(UTC)).total_seconds())
 
 
+# A shared session pools the keep-alive connection across the per-post sweep
+# calls, and retries CONNECT failures only (connect=2, short backoff) — smoothing
+# the instant a reverse proxy reloads. Status codes are deliberately NOT retried
+# (status=0, raise_on_status=False): translate() maps 429/5xx → InterpreterUnavailable
+# itself, and letting urllib3 retry a draining 503 would defeat the Retry-After
+# backoff we honour upstream.
+_retry = Retry(
+    total=None, connect=2, read=0, redirect=0, status=0,
+    backoff_factor=0.3, raise_on_status=False,
+)
+session = requests.Session()
+_adapter = HTTPAdapter(max_retries=_retry)
+session.mount("http://", _adapter)
+session.mount("https://", _adapter)
+
+
 def health(base_url: str, *, timeout: float = 5.0) -> bool:
     """True iff the Interpreter LLM engine is up. Any error (unset URL, network,
     non-200, engine down) → False, so the sweep just no-ops rather than raising."""
     if not base_url:
         return False
     try:
-        r = requests.get(_url(base_url, "/v1/health"), timeout=timeout)
+        r = session.get(_url(base_url, "/v1/health"), timeout=timeout)
     except requests.RequestException:
         return False
     if r.status_code != 200:
@@ -94,7 +112,7 @@ def translate(
         return {"translations": [], "detected_lang": None,
                 "engine": None, "engine_version": None}
     try:
-        r = requests.post(
+        r = session.post(
             _url(base_url, "/v1/translate"),
             json={"q": list(texts), "source": source,
                   "target": target, "engine": "auto"},