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/services/memories/immichService.ts

@@ -230,6 +230,30 @@ export async function getAssetInfo(
   }
 }
 
+export async function fetchImmichThumbnailBytes(
+  userId: number,
+  assetId: string,
+  ownerUserId?: number
+): Promise<{ bytes: Buffer; contentType: string } | { error: string; status: number }> {
+  const effectiveUserId = ownerUserId ?? userId;
+  const creds = getImmichCredentials(effectiveUserId);
+  if (!creds) return { error: 'Not found', status: 404 };
+
+  const url = `${creds.immich_url}/api/assets/${assetId}/thumbnail?size=thumbnail`;
+  try {
+    const resp = await safeFetch(url, {
+      headers: { 'x-api-key': creds.immich_api_key },
+      signal: AbortSignal.timeout(10000) as any,
+    });
+    if (!resp.ok) return { error: 'Upstream error', status: resp.status };
+    const contentType = resp.headers.get('content-type') || 'image/jpeg';
+    const bytes = Buffer.from(await resp.arrayBuffer());
+    return { bytes, contentType };
+  } catch {
+    return { error: 'Proxy error', status: 502 };
+  }
+}
+
 export async function streamImmichAsset(
   response: Response,
   userId: number,