feat: places (reverse geocoding) + folder browser

Places:
- worker/geocode.js: Nominatim reverse geocoder, 1 req/sec rate limit,
  in-memory cache keyed to 3-decimal-place coordinates
- Ingest calls reverseGeocode() for any photo with GPS data; stores
  place_city and place_country on the photo row
- DB migrations add place_city / place_country columns + index on place_city
- Stats API returns cities grouped by count
- buildFilter() supports ?city= parameter
- FilterSidebar shows a Places section with city list + country sublabel

Folder browser:
- GET /api/fs/browse?path=… lists subdirectories (path-normalized to prevent traversal)
- Settings panel: 📁 button opens inline directory browser below the add
  input — navigate with up arrow / directory clicks, Select fills the input

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

worker/geocode.js

@@ -0,0 +1,36 @@
+const cache    = new Map();
+let lastCallMs = 0;
+
+export async function reverseGeocode(lat, lon) {
+  const key = `${lat.toFixed(3)},${lon.toFixed(3)}`;
+  if (cache.has(key)) return cache.get(key);
+
+  // Nominatim ToS: max 1 request/second
+  const wait = 1100 - (Date.now() - lastCallMs);
+  if (wait > 0) await new Promise(r => setTimeout(r, wait));
+  lastCallMs = Date.now();
+
+  try {
+    const url = `https://nominatim.openstreetmap.org/reverse?lat=${lat}&lon=${lon}&format=json&zoom=10`;
+    const res = await fetch(url, {
+      headers: { 'User-Agent': 'Luma/1.0 self-hosted-photo-manager' },
+      signal:  AbortSignal.timeout(8000),
+    });
+    if (!res.ok) throw new Error(res.status);
+    const data = await res.json();
+    const place = {
+      city:    data.address?.city
+            ?? data.address?.town
+            ?? data.address?.village
+            ?? data.address?.county
+            ?? null,
+      country: data.address?.country ?? null,
+    };
+    cache.set(key, place);
+    return place;
+  } catch (err) {
+    console.warn(`[geocode] ${lat.toFixed(4)},${lon.toFixed(4)}: ${err.message}`);
+    cache.set(key, null);
+    return null;
+  }
+}