Add clear DTC codes button to Pi DTC page (FEAT-15)

tests/test_dtc_persistence.py

@@ -62,6 +62,22 @@ class DtcPersistenceTest(unittest.TestCase):
         self.assertEqual(app_sql.count_active_dtcs(), 0)
         self.assertIs(app_sql.get_persisted_dtc_state()["mil"], False)
 
+    def test_clear_dtc_state_clears_active_and_mil(self):
+        app_sql.save_dtc_state(True, [
+            {"code": "P0300", "description": "Random misfire"},
+            {"code": "P0420", "description": "Catalyst efficiency"},
+        ])
+        app_sql.clear_dtc_state()
+        state = app_sql.get_persisted_dtc_state()
+        self.assertIs(state["mil"], False)
+        self.assertEqual(state["codes"], [])
+        self.assertEqual(app_sql.count_active_dtcs(), 0)
+        # History rows are kept but no longer set.
+        rows = self.conn.execute(
+            "SELECT code, IS_SET FROM dtc ORDER BY code"
+        ).fetchall()
+        self.assertEqual(rows, [("P0300", 0), ("P0420", 0)])
+
     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)