fix(halacha): global advisory lock — one extraction at a time (prevents box freeze)

2026-05-31: opus-4-8 @ xhigh extraction + overlapping driver processes (agent
fallback retries each spawn an independent `python -c` driver; process_pending is
serial WITHIN a process but the box ran 4-5 drivers in parallel) → 12-16 concurrent
xhigh `claude -p` procs → load 69 → hard reboot.

Fix: halacha_extractor.extract() now takes a Postgres advisory lock
(pg_try_advisory_lock, key 'HALA') before any work. If another extraction (any
process/agent/driver — all share the legal-ai DB) holds it, the call returns
status='busy' and the precedent stays pending for the next drain. Guarantees ONE
extraction at a time ACROSS PROCESSES — an in-process Semaphore cannot (drivers
are separate OS processes). Core logic moved to _extract_impl (unchanged) under
the lock. CHUNK_CONCURRENCY now env-tunable (HALACHA_CHUNK_CONCURRENCY, default 3).

Verified: while a lock is held, extract() returns 'busy' with no LLM call; lock
releases cleanly and the next extraction proceeds. Tracks #72.

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

mcp-server/src/legal_mcp/config.py

@@ -54,6 +54,11 @@ REDIS_URL = os.environ.get("REDIS_URL", "redis://127.0.0.1:6380/0")
 # pinned.
 HALACHA_EXTRACT_MODEL = os.environ.get("HALACHA_EXTRACT_MODEL", "claude-opus-4-8")
 HALACHA_EXTRACT_EFFORT = os.environ.get("HALACHA_EXTRACT_EFFORT", "xhigh")
+# Concurrent chunks WITHIN a single extraction. Each `claude -p` @ xhigh holds
+# ~300MB RSS + heavy CPU; cross-process overlap (agent retries) on top of this
+# froze the box on 2026-05-31 (hard reboot). A global advisory lock now caps
+# the system to ONE extraction at a time; this caps the chunks within it.
+HALACHA_CHUNK_CONCURRENCY = int(os.environ.get("HALACHA_CHUNK_CONCURRENCY", "3"))
 HALACHA_CORROBORATION_MATCH_FLOOR = float(os.environ.get("HALACHA_CORROBORATION_MATCH_FLOOR", "0.50"))
 HALACHA_CORROBORATION_MIN_CITES = int(os.environ.get("HALACHA_CORROBORATION_MIN_CITES", "2"))