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).

tests/test_dtc_persistence.py

@@ -0,0 +1,109 @@
+import os
+import tempfile
+import unittest
+from datetime import datetime
+
+from modules import app_sql
+
+
+class DtcPersistenceTest(unittest.TestCase):
+    """Tests for persisted DTC/MIL state (BUG-03)."""
+
+    def setUp(self):
+        handle, self.path = tempfile.mkstemp(suffix=".db")
+        os.close(handle)
+        self.original_path = app_sql.sql_path
+        app_sql._conn = None
+        app_sql.sql_path = self.path
+        self.conn = app_sql.get_conn()
+
+    def tearDown(self):
+        if app_sql._conn is not None:
+            app_sql._conn.close()
+        app_sql._conn = None
+        app_sql.sql_path = self.original_path
+        os.unlink(self.path)
+
+    def test_save_and_read_back_dtc_state(self):
+        app_sql.save_dtc_state(True, [
+            {"code": "P0300", "description": "Random misfire"},
+            {"code": "P0420", "description": "Catalyst efficiency"},
+        ])
+        state = app_sql.get_persisted_dtc_state()
+        self.assertIs(state["mil"], True)
+        self.assertEqual(
+            [c["code"] for c in state["codes"]],
+            ["P0300", "P0420"],
+        )
+        self.assertEqual(state["codes"][0]["description"], "Random misfire")
+        self.assertEqual(app_sql.count_active_dtcs(), 2)
+
+    def test_cleared_code_removed_from_active(self):
+        app_sql.save_dtc_state(True, [
+            {"code": "P0300", "description": "Random misfire"},
+            {"code": "P0420", "description": "Catalyst efficiency"},
+        ])
+        # A later poll reports only P0420 -> P0300 is cleared.
+        app_sql.save_dtc_state(True, [
+            {"code": "P0420", "description": "Catalyst efficiency"},
+        ])
+        state = app_sql.get_persisted_dtc_state()
+        self.assertEqual([c["code"] for c in state["codes"]], ["P0420"])
+        self.assertEqual(app_sql.count_active_dtcs(), 1)
+        # The cleared code keeps its history row (IS_SET=0).
+        rows = self.conn.execute(
+            "SELECT code, IS_SET FROM dtc ORDER BY code"
+        ).fetchall()
+        self.assertEqual(rows, [("P0300", 0), ("P0420", 1)])
+
+    def test_empty_poll_clears_all(self):
+        app_sql.save_dtc_state(True, [{"code": "P0300", "description": "Misfire"}])
+        app_sql.save_dtc_state(False, [])
+        self.assertEqual(app_sql.count_active_dtcs(), 0)
+        self.assertIs(app_sql.get_persisted_dtc_state()["mil"], False)
+
+    def test_first_seen_preserved_while_last_seen_bumps(self):
+        first = datetime(2026, 2, 1, 8, 0, 0)
+        later = datetime(2026, 2, 1, 8, 5, 0)
+        app_sql.save_dtc_state(True, [{"code": "P0300", "description": "M"}], first)
+        app_sql.save_dtc_state(True, [{"code": "P0300", "description": "M"}], later)
+        row = self.conn.execute(
+            "SELECT FIRST_SEEN_AT, LAST_SEEN_AT FROM dtc WHERE code = 'P0300'"
+        ).fetchone()
+        self.assertEqual(row[0], first.isoformat())
+        self.assertEqual(row[1], later.isoformat())
+
+    def test_overview_stats_uses_dtc_table_not_broken_dtc_codes(self):
+        app_sql.save_dtc_state(True, [
+            {"code": "P0300", "description": "Random misfire"},
+        ])
+        stats = app_sql.get_overview_stats()
+        self.assertEqual(stats["active_dtc"], 1)
+
+    def test_old_schema_dtc_table_migrated(self):
+        # Simulate the original (never-populated) dtc schema.
+        app_sql._conn.execute("DROP TABLE dtc")
+        app_sql._conn.execute(
+            """CREATE TABLE dtc (
+                   id integer PRIMARY KEY,
+                   NAME text NOT NULL,
+                   STATUS text,
+                   DTC_ID text
+               )"""
+        )
+        app_sql._conn.commit()
+        app_sql._migrate_dtc_table(app_sql._conn)
+        columns = {
+            row[1] for row in app_sql._conn.execute("PRAGMA table_info(dtc)").fetchall()
+        }
+        self.assertIn("code", columns)
+        self.assertNotIn("DTC_ID", columns)
+
+    def test_no_persisted_state_returns_empty(self):
+        state = app_sql.get_persisted_dtc_state()
+        self.assertIsNone(state["mil"])
+        self.assertEqual(state["codes"], [])
+
+
+if __name__ == "__main__":
+    unittest.main()