fix(db): serialise schema migrations with an advisory lock + stagger drain crons

legal-halacha-drain crashed 29× with asyncpg DeadlockDetectedError. Root cause:
every short-lived cron drain re-runs the idempotent schema migrations on startup
(get_pool → _run_schema_migrations), and three jobs (metadata-drain, halacha-drain,
halacha-supervisor) all fired on the same minute (*/15 / top-of-hour). Two
processes running the DDL concurrently took AccessExclusiveLock in opposite order
→ Postgres killed one with a deadlock.

Two-layer fix:
- Root cause: wrap _run_schema_migrations in a session-level pg_advisory_lock so
  only one process applies DDL at a time; concurrent migrators wait instead of
  deadlocking. DDL body extracted to _apply_schema_ddl. Idempotent, schema
  unchanged.
- Defence-in-depth: give each cron drain a distinct firing minute —
  metadata :00, supervisor :05, halacha-drain :10, digest :12, court-fetch :17 —
  so siblings no longer start at the same instant. SCRIPTS.md updated to match.

Invariants: G1 (fix at source — the single migration path — not the symptom);
G2 (no parallel control path introduced).

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

scripts/legal-digest-drain.config.cjs

@@ -17,9 +17,12 @@
  *     pm2 start /home/chaim/legal-ai/scripts/legal-digest-drain.config.cjs
  *     pm2 save
  * Run now (manual): mcp-server/.venv/bin/python scripts/drain_digests.py
- * Schedule override: DIGEST_DRAIN_CRON (default every 2 h at :00).
+ * Schedule override: DIGEST_DRAIN_CRON (default every 2 h at :12).
  */
-const cron = process.env.DIGEST_DRAIN_CRON || "0 */2 * * *";
+// Minute :12 (not :00) so it never shares a firing minute with the every-hour
+// legal-metadata-drain (:00) — avoids the schema-migration DDL deadlock when
+// sibling drains start at the same instant.
+const cron = process.env.DIGEST_DRAIN_CRON || "12 */2 * * *";
 
 module.exports = {
   apps: [