embeddings: register pool close() at interpreter exit

Without this, every `second-brain process` run prints
  couldn't stop thread 'pool-1-worker-N' within 5.0 seconds
during interpreter shutdown — psycopg_pool's background workers don't
get a clean stop signal before Python's threading shutdown deadline.

Registering close_pool via atexit the first time we open the pool fixes
it without changing any API. `close_pool` is already idempotent, so the
explicit teardown path in the smoke test (which calls it directly) and
the atexit path coexist safely.

src/second_brain/embeddings/__init__.py

@@ -23,6 +23,7 @@ index, not authoritative state.
 
 from __future__ import annotations
 
+import atexit
 import logging
 import os
 from typing import Any
@@ -73,6 +74,11 @@ def _get_pool() -> ConnectionPool | None:
     if not dsn:
         return None
     _pool = ConnectionPool(dsn, min_size=1, max_size=4, open=True, timeout=10)
+    # Make sure the pool's worker threads get a clean shutdown when the
+    # interpreter exits; without this psycopg_pool prints
+    # "couldn't stop thread pool-1-worker-N within 5.0 seconds" on every
+    # `second-brain process` run.
+    atexit.register(close_pool)
     return _pool