Implements the clear-history endpoint required by issue #3. All other
pieces (session_messages table, getSessionHistory, context injection,
frontend clear button) were already in place — this adds the missing
backend route that the frontend's _clearHistory function calls.
- DELETE /sessions/:id/history requires valid macaroon (Bearer auth)
- Returns 404 if session not found, 401 on bad macaroon, 410 if expired
- Deletes all session_messages rows for the session on success
Fixes#3
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
