fix(phase26): persist DTC/MIL state + server speed_source (BUG-03, BUG-05)

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).

app.py

@@ -335,9 +335,17 @@ def data():
 
 @app.route('/api/dtc')
 def api_dtc():
+    fresh_mil = app_obd.get_fresh_mil()
+    fresh_codes = app_obd.get_fresh_dtc_codes()
+    if fresh_mil is None and not fresh_codes:
+        # OBD offline/stale (disconnected or not yet polled): fall back to the
+        # last-persisted DTC snapshot so codes survive a disconnect/reboot.
+        persisted = app_sql.get_persisted_dtc_state()
+        fresh_mil = persisted["mil"]
+        fresh_codes = persisted["codes"]
     return jsonify({
-        "mil": app_obd.get_fresh_mil(),
-        "codes": app_obd.get_fresh_dtc_codes(),
+        "mil": fresh_mil,
+        "codes": fresh_codes,
     })
 
 @app.route('/api/stats')