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

client/src/components/Journey/JourneyMap.tsx

@@ -14,6 +14,7 @@ export interface MapMarkerItem {
 export interface JourneyMapHandle {
   highlightMarker: (id: string | null) => void
   focusMarker: (id: string) => void
+  invalidateSize: () => void
 }
 
 interface MapEntry {
@@ -151,7 +152,11 @@ const JourneyMap = forwardRef<JourneyMapHandle, Props>(function JourneyMap(
     }
   }, [])
 
-  useImperativeHandle(ref, () => ({ highlightMarker, focusMarker }), [])
+  const invalidateSize = useCallback(() => {
+    try { mapRef.current?.invalidateSize() } catch { /* map not yet initialized */ }
+  }, [])
+
+  useImperativeHandle(ref, () => ({ highlightMarker, focusMarker, invalidateSize }), [])
 
   useEffect(() => {
     if (!containerRef.current) return