[Backend] Create /agent/create API with core_memory

## Input
- name (str, 1-20)
- age (int)
- personality (str, max 200)
- catchphrase (str, max 50)

## Logic
- Verify session → get user_id
- Generate agent_id = "agnt_" + uuid4()
- Insert into agents table (SQLite)
- Return { "agent_id": "...", "status": "created" }

## Error
- 401 if not logged in
- 400 if field invalid

## Tech
- Use FastAPI + aiosqlite
- Reuse get_current_user() dependency