feat(atlas): sub-national region view when zooming in

- Zoom >= 5 shows visited regions (states/provinces/departments) colored on the map
- Server resolves places to regions via Nominatim reverse geocoding (zoom=8)
- Supports all ISO levels: lvl4 (states), lvl5 (provinces), lvl6 (departments)
- Handles city-states (Berlin, Vienna, Hamburg) via city/county fallback
- Fuzzy name matching between Nominatim and GeoJSON for cross-format compatibility
- 10m admin_1 GeoJSON loaded server-side (cached), filtered per country
- Region colors match their parent country color
- Custom DOM tooltip (ref-based, no re-renders on hover)
- Country layer dims to 35% opacity when regions visible
- place_regions DB table caches resolved regions permanently
- Rate-limited Nominatim calls (1 req/sec) with progressive resolution

server/src/db/migrations.ts

@@ -550,6 +550,19 @@ function runMigrations(db: Database.Database): void {
         );
       `);
     },
+    // Migration 69: Place region cache for sub-national Atlas regions
+    () => {
+      db.exec(`
+        CREATE TABLE IF NOT EXISTS place_regions (
+          place_id INTEGER PRIMARY KEY REFERENCES places(id) ON DELETE CASCADE,
+          country_code TEXT NOT NULL,
+          region_code TEXT NOT NULL,
+          region_name TEXT NOT NULL
+        );
+        CREATE INDEX IF NOT EXISTS idx_place_regions_country ON place_regions(country_code);
+        CREATE INDEX IF NOT EXISTS idx_place_regions_region ON place_regions(region_code);
+      `);
+    },
   ];
 
   if (currentVersion < migrations.length) {