perf: geocode pipeline — batch POST + parallel server, 200→1 roundtrip

JS preload(): replaced N individual GET /api/geocode (concurrency 2) with
a single POST /api/geocode/batch — one HTTP roundtrip instead of up to
N/2 sequential ones. schedLoc debounce reduced 600→200ms.

Server geocode_batch(): ThreadPoolExecutor(max_workers=4) parallelises
Nominatim/reverse_geocoder calls instead of sequential loop. Deduplicates
coords before submitting futures. lru_cache absorbs duplicates within batch.

Pi geocode_batch_route(): deduplicate coords, removed stale 'if label:'
guard (label always non-empty after BUG-13 fix).

server/app.py

@@ -2,6 +2,7 @@
 uploaded by the Pi, storing them in Postgres + on-disk file storage.
 Also serves the remote web portal for browsing and managing the configured car.
 """
+import concurrent.futures
 import functools
 import hashlib
 import os
@@ -626,12 +627,20 @@ def geocode():
 @app.post("/api/geocode/batch")
 @require_login
 def geocode_batch():
-    """Batch geocode — returns {key: label} for all unique coords in one request."""
+    """Batch geocode — returns {key: label} for all unique coords in one request.
+
+    Parallelised with ThreadPoolExecutor (max_workers=4) so Nominatim HTTP
+    calls and reverse_geocoder lookups run concurrently instead of sequentially.
+    _geocode_cached's lru_cache absorbs duplicate coords within the batch."""
     body = request.get_json(silent=True) or {}
     coords = body.get("coords")
     if not isinstance(coords, list) or not coords:
         return jsonify(error="expected {coords: [{lat, lon}, ...]}"), 400
-    result = {}
+
+    # Deduplicate — same (lat4,lon4) would hit lru_cache anyway, but this
+    # avoids submitting redundant futures.
+    seen = set()
+    items = []
     for item in coords:
         try:
             lat = float(item["lat"])
@@ -640,9 +649,25 @@ def geocode_batch():
             continue
         lat4 = int(round(lat, 4) * 10000)
         lon4 = int(round(lon, 4) * 10000)
-        key = f"{lat:.3f},{lon:.3f}"
-        name = _geocode_cached(lat4, lon4)
-        result[key] = name
+        if (lat4, lon4) not in seen:
+            seen.add((lat4, lon4))
+            items.append((f"{lat:.3f},{lon:.3f}", lat4, lon4))
+
+    result = {}
+    if not items:
+        return jsonify(result)
+
+    with concurrent.futures.ThreadPoolExecutor(max_workers=4) as ex:
+        futures = {ex.submit(_geocode_cached, lat4, lon4): key for key, lat4, lon4 in items}
+        for future in concurrent.futures.as_completed(futures):
+            key = futures[future]
+            try:
+                result[key] = future.result(timeout=8)
+            except Exception:
+                # Last resort — raw coords if even the fallback failed
+                parts = key.split(",")
+                result[key] = f"{float(parts[0]):.5f}, {float(parts[1]):.5f}"
+
     return jsonify(result)