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

FEATURES.md

@@ -34,6 +34,7 @@ dedicated PLAN.md phase (or fold it into the relevant existing work) and mark it
 | FEAT-12 | `SHIPPED` | Log OBD speed before GPS fix (rolling-start capture) | Phase 27 |
 | FEAT-13 | `IDEA` | Flush uploaded tracking rows from local DB | unassigned |
 | FEAT-14 | `IDEA` | Second T3U to replace onboard wlan0 (dual AC radios) | unassigned |
+| FEAT-15 | `SHIPPED` | Clear DTC codes from the DTC page | Phase 29 |
 
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@@ -275,3 +276,17 @@ dedicated PLAN.md phase (or fold it into the relevant existing work) and mark it
   - The rtl88x2bu driver is already built; a second identical dongle should be plug-and-play.
 
 
+### FEAT-15 — Clear DTC codes from the DTC page
+- **Status**: `SHIPPED` 2026-08-13
+- **Area**: `modules/app_obd.py`, `modules/app_sql.py`, `app.py`, `templates/dtc.html`
+- **Phase**: Phase 29
+- **What**: The Pi `/dtc` page now has a **Clear codes** button that sends OBD mode 04
+  (`CLEAR_DTC`) to the ECU and resets both the live and persisted DTC state, so codes can be
+  cleared right after a repair without a separate scanner. If the adapter isn't connected to
+  the car ECU it reports "not connected" (409), and if a fault is still physically present the
+  codes legitimately re-appear on the next sensor poll.
+- **Files**: `clear_dtc_codes()` in `app_obd.py` (mode 04 + in-memory/socket reset),
+  `clear_dtc_state()` in `app_sql.py` (marks active rows `IS_SET=0`, resets `dtc_mil`),
+  `POST /api/dtc/clear` in `app.py`, and the button + confirm/status JS in `templates/dtc.html`.
+
+