BUG-03: DTC/MIL codes were never persisted; lost on OBD disconnect or reboot.
- Add new dtc table schema (code, description, MIL, first/last seen, IS_SET)
and a migration that rebuilds the original unused NAME/STATUS/DTC_ID table.
- Persist MIL + codes to SQLite on each successful OBD poll (_persist_dtc_state).
- /api/dtc falls back to the last-persisted snapshot when OBD is offline/stale.
- Fix get_overview_stats to count the real dtc table (was querying non-existent
dtc_codes table).
- Add cleared/since tracking via IS_SET + FIRST_SEEN_AT/LAST_SEEN_AT.
BUG-05: server trip replay hardcoded speed_source:'gps'; mirror Pi's
'obd if speed>0 else gps' logic in server get_trip_route.
Adds tests/test_dtc_persistence.py (7 tests).
Mathieu Moran