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

modules/app_sql.py

@@ -280,6 +280,33 @@ def save_dtc_state(mil_on, dtc_codes, recorded_at=None):
             print("Failed to persist DTC state:", e)
 
 
+def clear_dtc_state(recorded_at=None):
+    """Mark every active DTC as cleared and reset the persisted MIL flag.
+
+    Called after the user explicitly clears codes on the ECU (OBD mode 04) so
+    the persisted snapshot stops showing stale active codes. Cleared codes
+    keep their history rows (IS_SET = 0), like a normal "no longer detected"
+    poll would.
+    """
+    conn = get_conn()
+    if conn is None:
+        return
+    now_str = (recorded_at or datetime.now()).isoformat()
+    with _conn_lock:
+        try:
+            conn.execute(
+                "INSERT INTO settings (key, value) VALUES ('dtc_mil', '0') "
+                "ON CONFLICT(key) DO UPDATE SET value = excluded.value"
+            )
+            conn.execute(
+                "UPDATE dtc SET IS_SET = 0, LAST_SEEN_AT = ? WHERE IS_SET = 1",
+                (now_str,),
+            )
+            conn.commit()
+        except sqlite3.Error as e:
+            print("Failed to clear DTC state:", e)
+
+
 def get_persisted_dtc_state():
     """Return the last-persisted MIL + DTC codes for display when OBD is offline."""
     conn = get_conn()