Large albums opened slowly and flickered; the default album cover was
also lost in the media-objects refactor.
Large-album loading:
- quantize signed object URLs within a window and send Cache-Control so
previews are reused instead of re-fetched
- publish photo.updated (not photo.created) when variants are ready, so
clients update in place without reordering the grid
- worker concurrency + newest-first variant priority; raise default batch
Album cover:
- first uploaded photo becomes the cover when none is set
- clear the cover when the cover photo is trashed or moved out
- "set as album cover" in the photo context menu and actions panel
OpenSpec changes: fix-large-album-load, fix-album-cover
- introduce storage.Backend with S3 and local file-system implementations
- deliver all blobs via signed GET/HEAD /api/objects/<key> (Range, Content-Type)
- stream S3 objects through the backend; remove presigned URL path
- share shared HMAC Signer between backends
- remove S3_PUBLIC_URL; rename S3_PRESIGN_TTL to OBJECT_URL_TTL
- make storage label in health dynamic; nginx proxy_buffering off for objects
iu