feat: switch notification backend from Redis to Firestore

- Refactor FirestoreNotificationBackend to use time-gated window
  (window_hours) instead of notified_by_agent status filtering;
  mark_as_notified is now a no-op (agent is awareness-only).
- Update agent.py to instantiate FirestoreNotificationBackend using the
  shared firestore_db client instead of RedisNotificationBackend.
- Remove redis_host/redis_port settings from config.py; add
  notifications_collection_path, max_to_notify, and window_hours.
- Move redis/json imports inside RedisNotificationBackend methods so
  redis is only required if that backend is explicitly instantiated.
- Add utility scripts for checking and registering notifications.
- Add google-cloud-firestore dependency to pyproject.toml.

src/va_agent/config.py

@@ -31,6 +31,7 @@ class AgentSettings(BaseSettings):
         "artifacts/bnt-orquestador-cognitivo-dev/notifications"
     )
     notifications_max_to_notify: int = 5
+    notifications_window_hours: float = 48
 
     # MCP configuration
     mcp_audience: str