feat: MercadoLibre integration + inventory bulk publish + WhatsApp bridge fixes

- Add MercadoLibre OAuth, listings, orders, webhooks and category search
- New marketplace_external_bp.py, meli_service.py, marketplace_external_service.py
- New marketplace_external.html/js with ML management UI
- Inventory: bulk publish to ML with category autocomplete, listing type and shipping selectors
- Inventory: new .btn--meli styles, select/label CSS fixes
- WhatsApp bridge: rate limiting, 440/515/408 error handling, stale watchdog
- DB migration v3.4_meli_integration.sql for marketplace_listings, orders, sync_queue
- Add Celery tasks for ML sync and webhook processing
- Sidebar: MercadoLibre navigation link

pos/services/geo_branches.py

@@ -0,0 +1,56 @@
+import math
+
+def haversine(lat1, lon1, lat2, lon2):
+    """Calculate the great-circle distance between two points on Earth in km."""
+    R = 6371.0  # Earth radius in km
+    phi1 = math.radians(lat1)
+    phi2 = math.radians(lat2)
+    dphi = math.radians(lat2 - lat1)
+    dlambda = math.radians(lon2 - lon1)
+
+    a = math.sin(dphi / 2) ** 2 + math.cos(phi1) * math.cos(phi2) * math.sin(dlambda / 2) ** 2
+    c = 2 * math.atan2(math.sqrt(a), math.sqrt(1 - a))
+
+    return R * c
+
+
+def find_nearest_branch(tenant_conn, latitude, longitude):
+    """
+    Find the nearest active branch with coordinates.
+    Returns a dict with branch info + distance_km, or None.
+    """
+    if not tenant_conn or latitude is None or longitude is None:
+        return None
+
+    cur = tenant_conn.cursor()
+    cur.execute(
+        """
+        SELECT id, name, address, phone, latitude, longitude
+        FROM branches
+        WHERE is_active = TRUE AND latitude IS NOT NULL AND longitude IS NOT NULL
+        """
+    )
+    branches = cur.fetchall()
+    cur.close()
+
+    nearest = None
+    min_dist = float('inf')
+
+    for row in branches:
+        bid, name, address, phone, b_lat, b_lon = row
+        if b_lat is None or b_lon is None:
+            continue
+        dist = haversine(float(latitude), float(longitude), float(b_lat), float(b_lon))
+        if dist < min_dist:
+            min_dist = dist
+            nearest = {
+                'id': bid,
+                'name': name,
+                'address': address or '',
+                'phone': phone or '',
+                'latitude': float(b_lat),
+                'longitude': float(b_lon),
+                'distance_km': round(dist, 1),
+            }
+
+    return nearest