* fix(auth): prune expired sessions on every verify to prevent memory leak
The in-memory _sessions dict accumulated expired tokens indefinitely —
entries were only removed when that specific token was verified. Add a
lazy _prune_expired_sessions() call at the top of verify_session() so
all expired entries are swept during normal traffic.
Addresses #192.
* test(auth): add 8 unit tests for session lifecycle and lazy pruning
Tests verify:
- Fresh session creation and validation
- Expired entries are pruned during verify_session() calls
- Valid sessions are never removed by pruning
- Empty dict is safe for pruning
- Session TTL matches expected 24-hour window
- invalidate_session() actually removes the token
- Invalidating non-existent tokens is safe
