refactor: ditch Nominatim, use reverse_geocoder exclusively for geocoding

Nominatim was slow (rate-limited, 1 req/sec, ~4s timeouts) and unreliable
from Docker/Cloudflare (blocked or throttled). It was the root cause of
location labels failing to appear during trip scrubbing.

- server/app.py: _geocode_cached now uses reverse_geocoder exclusively
  (offline, instant, city/province level). Removed Nominatim HTTP calls,
  ThreadPoolExecutor, concurrent.futures import.
- app.py (Pi): same — reverse_geocoder imported at module level,
  Nominatim + urllib.request removed.
- Batch endpoint simplified to sequential loop with lru_cache + dedup.
  No network dependency at all — geocoding is instant and always works.

reverse_geocoder provides city/province labels (e.g. 'Montreal, Quebec')
which is the right granularity for trip scrubbing.

app.py

@@ -163,36 +163,23 @@ def api_delete_trip(trip_id):
     return jsonify(status="deleted"), 200
 
 import functools
-import urllib.request
+import reverse_geocoder as rg
 
 @functools.lru_cache(maxsize=512)
 def _geocode_cached(lat4, lon4):
-    """Try Nominatim first (street-level), fall back to reverse_geocoder (city),
-    then raw coordinates. Always returns a non-empty label string."""
+    """Reverse-geocode a coordinate to city/province using the offline
+    reverse_geocoder library. Always returns a non-empty label string."""
     lat = lat4 / 10000.0
     lon = lon4 / 10000.0
     try:
-        url = f"https://nominatim.openstreetmap.org/reverse?format=jsonv2&lat={lat}&lon={lon}&accept-language=en"
-        req = urllib.request.Request(url, headers={"User-Agent": "yaris-obd/1.0"})
-        with urllib.request.urlopen(req, timeout=4) as resp:
-            data = json.loads(resp.read())
-        a = data.get("address", {})
-        road = a.get("road") or a.get("hamlet") or a.get("suburb") or ""
-        city = a.get("city") or a.get("town") or a.get("village") or a.get("municipality") or ""
-        label = ", ".join(p for p in [road, city] if p)
-        if label:
-            return label
+        results = rg.search([(lat, lon)], mode=1, verbose=False)
+        if results:
+            r = results[0]
+            label = ", ".join(p for p in [r.get("name", ""), r.get("admin1", "")] if p)
+            if label:
+                return label
     except Exception:
         pass
-    # Offline fallback
-    import reverse_geocoder as rg
-    results = rg.search([(lat, lon)], mode=1, verbose=False)
-    if results:
-        r = results[0]
-        label = ", ".join(p for p in [r.get("name", ""), r.get("admin1", "")] if p)
-        if label:
-            return label
-    # Last resort — raw coordinates so the frontend never gets an empty label
     return f"{lat:.5f}, {lon:.5f}"
 
 @app.route('/api/geocode')