feat: FEAT-05 — DTC history sync to remote server

- Server: dtc_events table (migrate_phase27_dtc.sql), db.py helpers
  (upsert_dtc_snapshot, list_dtc_events, delete_dtc_event)
- Server API: POST /api/v1/dtc/batch (device auth), GET/DELETE /api/v1/portal/dtc
- Server portal: /dtc page with code, description, MIL, status, timestamps, delete
- Server nav: DTC history in sidebar + mobile bottom nav
- Pi: _upload_dtc_snapshot() in app_upload.py — uploads when snapshot changes,
  skips when OBD disconnected or unchanged

modules/app_upload.py

@@ -310,6 +310,50 @@ def _upload_dashcam_files():
     return uploaded_count
 
 
+_last_dtc_hash = None  # tracks last-uploaded DTC snapshot to avoid redundant pushes
+
+
+def _upload_dtc_snapshot():
+    """Upload current DTC snapshot to server if it differs from last upload.
+    Skips when OBD is disconnected (no point sending empty snapshots that
+    would clear all server-side DTC events)."""
+    global _last_dtc_hash
+    import hashlib as _hl
+    from . import app_obd, app_sql
+
+    # Only upload when OBD is actually connected — don't clear server DTCs
+    # just because the Pi rebooted or the adapter is unplugged.
+    if app_obd.connection is None:
+        return
+
+    mil_on = app_obd.mil_on
+    codes = app_obd.dtc_codes or []
+    if not codes and mil_on is None:
+        return  # No data yet
+
+    # Hash the current snapshot to detect changes
+    payload = {"mil_on": bool(mil_on), "codes": codes}
+    h = _hl.sha256(repr(payload).encode()).hexdigest()
+    if h == _last_dtc_hash:
+        return
+    _last_dtc_hash = h
+
+    try:
+        creds = _credentials()
+        if not creds["server_url"] or not creds["device_name"] or not creds["api_key"]:
+            return
+        resp = requests.post(
+            f"{creds['server_url']}/api/v1/dtc/batch",
+            json=payload,
+            headers=_headers(creds),
+            timeout=5,
+        )
+        resp.raise_for_status()
+    except requests.RequestException:
+        _last_dtc_hash = None  # retry next cycle
+        raise
+
+
 def _upload_loop():
     global _last_error, _last_success_at
     import datetime as _dt
@@ -323,6 +367,7 @@ def _upload_loop():
             sent = _upload_tracking_batch()
             uploaded = _upload_dashcam_files()
             cleanup = app_video.cleanup_uploaded_videos()
+            _upload_dtc_snapshot()  # silently skips when unchanged or OBD offline
             if sent or uploaded:
                 print(f"Upload: sent {sent} tracking rows, {uploaded} dashcam files")
             if cleanup["deleted"]: