fix: sanitize FTS5 queries and close mirror DB connections

Two bugs fixed:

1. search_messages() crashes with OperationalError when user queries
   contain FTS5 special characters (+, ", (, {, dangling AND/OR, etc).
   Added _sanitize_fts5_query() to strip dangerous operators and a
   fallback try-except for edge cases.

2. _append_to_sqlite() in mirror.py creates a new SessionDB per call
   but never closes it, leaking SQLite connections. Added finally block
   to ensure db.close() is always called.

gateway/mirror.py

@@ -111,6 +111,7 @@ def _append_to_jsonl(session_id: str, message: dict) -> None:
 
 def _append_to_sqlite(session_id: str, message: dict) -> None:
     """Append a message to the SQLite session database."""
+    db = None
     try:
         from hermes_state import SessionDB
         db = SessionDB()
@@ -121,3 +122,6 @@ def _append_to_sqlite(session_id: str, message: dict) -> None:
         )
     except Exception as e:
         logger.debug("Mirror SQLite write failed: %s", e)
+    finally:
+        if db is not None:
+            db.close()