feat(atlas): mark sub-national regions as visited with cascade behavior

- Add visited_regions table migration
- Mark/unmark region endpoints with auto-mark parent country
- Unmark country cascades to its regions; unmark last region cascades to country
- Region modal with mark/unmark flow and bucket list shortcut
- Viewport-based lazy loading of region GeoJSON at zoom >= 6
- i18n: add atlas.markRegionVisitedHint and atlas.confirmUnmarkRegion across all 13 locales

server/src/db/migrations.ts

@@ -563,6 +563,20 @@ function runMigrations(db: Database.Database): void {
         CREATE INDEX IF NOT EXISTS idx_place_regions_region ON place_regions(region_code);
       `);
     },
+    () => {
+      db.exec(`
+        CREATE TABLE IF NOT EXISTS visited_regions (
+          id INTEGER PRIMARY KEY AUTOINCREMENT,
+          user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+          region_code TEXT NOT NULL,
+          region_name TEXT NOT NULL,
+          country_code TEXT NOT NULL,
+          created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+          UNIQUE(user_id, region_code)
+        );
+        CREATE INDEX IF NOT EXISTS idx_visited_regions_country ON visited_regions(country_code);
+      `);
+    },
   ];
 
   if (currentVersion < migrations.length) {