feat(photos): add 1h disk cache for remote thumbnails and keep tabs mounted

Closes #686

- Add trekPhotoCache service: SHA1-keyed disk cache under uploads/photos/trek/,
  1h TTL, in-flight dedup map to prevent stampedes on concurrent requests
- Add migration 108: trek_photo_cache_meta table
- Hook cache into streamPhoto for Immich/Synology thumbnail path;
  originals bypass cache
- Add fetchImmichThumbnailBytes / fetchSynologyThumbnailBytes returning
  Buffer instead of piping, used by the cache layer
- Add scheduler entry (every 2h + startup sweep) to evict expired disk
  files and DB rows via sweepExpired()
- Client: convert journey tab conditional-mount to hidden-toggle so
  img elements stay in DOM across tab switches, preventing redundant
  thumbnail requests on rapid tab changes
- Expose invalidateSize() on JourneyMapHandle; call it on map tab
  activation to fix Leaflet rendering in previously-hidden container

server/src/db/migrations.ts

@@ -1673,6 +1673,15 @@ function runMigrations(db: Database.Database): void {
           )
       `);
     }},
+    // Migration 108: Disk cache metadata for remote-provider photo thumbnails (Immich / Synology)
+    () => db.exec(`
+      CREATE TABLE IF NOT EXISTS trek_photo_cache_meta (
+        cache_key  TEXT    PRIMARY KEY,
+        content_type TEXT  NOT NULL DEFAULT 'image/jpeg',
+        fetched_at INTEGER NOT NULL
+      );
+      CREATE INDEX IF NOT EXISTS idx_trek_photo_cache_meta_fetched_at ON trek_photo_cache_meta (fetched_at);
+    `),
   ];
 
   if (currentVersion < migrations.length) {