fix(backend/copilot): strip secrets from graph data in _enrich_agents_with_graph

- Pass `for_export=True` to `get_graph()` so `stripped_for_export()`
  filters credentials, api_key, password, token, secret fields from
  `input_default` before the graph reaches the LLM context
- Use `agent.graph_version` (active version) instead of `version=None`
  to avoid exposing draft/unpublished graph versions
- Add `asyncio.timeout(15)` around `asyncio.gather` to prevent
  indefinite blocking on hung DB connections
- Resolve `graph_db()` once before the gather instead of per-coroutine
- Drop `get_graph_db` alias in favor of `graph_db` to match codebase

Fixes the CRITICAL security finding from autogpt-pr-reviewer.