feat(photos): full EXIF metadata extraction, backfill and info panel

- backend: switch EXIF parser to dsoprea/go-exif/v3 (all recognised tags, numeric normalization); store in photos.exif jsonb (migration 000003); add storage.Get and cmd/backfill-exif to backfill existing photos from S3 originals
- frontend: info panel renders all EXIF attributes with RU labels, enum translation and units (1/N, f/N, mm, EV); service tags hidden
- viewer: remove make-cover action (cover managed from panel only), icon-only download
- openspec: sync lightroom-workspace-ui artifacts (EXIF, cover-from-panel) and add plan for configurable info attributes/labels stored in user profile

backend/internal/models/models.go

@@ -75,8 +75,9 @@ type Photo struct {
 	TakenAt          *time.Time `json:"taken_at,omitempty"`
 	Latitude         *float64   `json:"latitude,omitempty"`
 	Longitude        *float64   `json:"longitude,omitempty"`
-	Caption          string     `json:"caption"`
-	CreatedAt        time.Time  `json:"created_at"`
+	Caption          string         `json:"caption"`
+	EXIF             map[string]any `json:"exif,omitempty"`
+	CreatedAt        time.Time      `json:"created_at"`
 	Uploader         PublicUser `json:"uploader"`
 	LikesCount       int        `json:"likes_count"`
 	CommentsCount    int        `json:"comments_count"`