Bug: SLM a2a_card_fetcher crashes — DatabaseService missing get_session

## Problem

SLM service `a2a_card_fetcher` logs error every 5 minutes:
```
services.a2a_card_fetcher - ERROR - A2A card refresh error: 'DatabaseService' object has no attribute 'get_session'
```

## Location
`autobot-slm-backend/services/a2a_card_fetcher.py`

## Discovered During
Working on #1809 (Docker containerization)

## Impact
**Low** — Non-fatal, SLM runs fine without it. Pollutes logs.