Fix L2 deployment: healthcheck, DB deadlock, CI image resolution

- Add /health endpoint (returns 200, skips auth middleware)
- Healthcheck now hits /health instead of / (which 302s to OAuth)
- Advisory lock in db.init_pool() prevents deadlock when 4 uvicorn
  workers race to run schema DDL
- CI: --resolve-image always on docker stack deploy to force re-pull

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

l2/db.py

@@ -187,9 +187,16 @@ async def init_pool():
         max_size=10,
         command_timeout=60
     )
-    # Create tables if they don't exist
+    # Create tables if they don't exist (advisory lock prevents deadlock
+    # when multiple uvicorn workers start simultaneously)
     async with _pool.acquire() as conn:
-        await conn.execute(SCHEMA)
+        acquired = await conn.fetchval("SELECT pg_try_advisory_lock(42)")
+        if acquired:
+            try:
+                await conn.execute(SCHEMA)
+            finally:
+                await conn.execute("SELECT pg_advisory_unlock(42)")
+        # If another worker holds the lock, schema is being created — skip
 
 
 async def close_pool():