fix(photos): faster large-album loading and album cover lifecycle

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

backend/internal/config/config.go

@@ -48,16 +48,19 @@ type SSE struct {
 }
 
 type Worker struct {
-	Interval   time.Duration
-	Batch      int
-	PendingTTL time.Duration
+	Interval    time.Duration
+	Batch       int
+	PendingTTL  time.Duration
+	Concurrency int
 }
 
 type Storage struct {
-	Backend   string
-	FSRoot    string
-	URLSecret string
-	URLTTL    time.Duration
+	Backend       string
+	FSRoot        string
+	URLSecret     string
+	URLTTL        time.Duration
+	URLSignWindow time.Duration
+	CacheMaxAge   time.Duration
 }
 
 type Database struct {
@@ -103,6 +106,16 @@ func Load() (*Config, error) {
 		return nil, fmt.Errorf("invalid OBJECT_URL_TTL: %w", err)
 	}
 
+	urlSignWindow, err := time.ParseDuration(getEnv("OBJECT_URL_SIGN_WINDOW", "10m"))
+	if err != nil {
+		return nil, fmt.Errorf("invalid OBJECT_URL_SIGN_WINDOW: %w", err)
+	}
+
+	cacheMaxAge, err := time.ParseDuration(getEnv("OBJECT_CACHE_MAX_AGE", "1h"))
+	if err != nil {
+		return nil, fmt.Errorf("invalid OBJECT_CACHE_MAX_AGE: %w", err)
+	}
+
 	heartbeat, err := time.ParseDuration(getEnv("SSE_HEARTBEAT_INTERVAL", "25s"))
 	if err != nil {
 		return nil, fmt.Errorf("invalid SSE_HEARTBEAT_INTERVAL: %w", err)
@@ -123,7 +136,7 @@ func Load() (*Config, error) {
 		return nil, fmt.Errorf("invalid WORKER_INTERVAL: %w", err)
 	}
 
-	workerBatch, err := strconv.Atoi(getEnv("WORKER_BATCH", "10"))
+	workerBatch, err := strconv.Atoi(getEnv("WORKER_BATCH", "100"))
 	if err != nil {
 		return nil, fmt.Errorf("invalid WORKER_BATCH: %w", err)
 	}
@@ -133,6 +146,11 @@ func Load() (*Config, error) {
 		return nil, fmt.Errorf("invalid WORKER_PENDING_TTL: %w", err)
 	}
 
+	workerConcurrency, err := strconv.Atoi(getEnv("WORKER_CONCURRENCY", "4"))
+	if err != nil {
+		return nil, fmt.Errorf("invalid WORKER_CONCURRENCY: %w", err)
+	}
+
 	logLevel := getEnv("LOG_LEVEL", "info")
 	if _, err := logging.ParseLevel(logLevel); err != nil {
 		return nil, err
@@ -160,10 +178,12 @@ func Load() (*Config, error) {
 			UsePathStyle: pathStyle,
 		},
 		Storage: Storage{
-			Backend:   getEnv("STORAGE_BACKEND", "fs"),
-			FSRoot:    getEnv("FS_ROOT", "./data"),
-			URLSecret: getEnv("URL_SECRET", ""),
-			URLTTL:    urlTTL,
+			Backend:       getEnv("STORAGE_BACKEND", "fs"),
+			FSRoot:        getEnv("FS_ROOT", "./data"),
+			URLSecret:     getEnv("URL_SECRET", ""),
+			URLTTL:        urlTTL,
+			URLSignWindow: urlSignWindow,
+			CacheMaxAge:   cacheMaxAge,
 		},
 		Database: Database{
 			Backend:    getEnv("DB_BACKEND", "sqlite"),
@@ -182,9 +202,10 @@ func Load() (*Config, error) {
 			ClientBuffer:      bufSize,
 		},
 		Worker: Worker{
-			Interval:   workerInterval,
-			Batch:      workerBatch,
-			PendingTTL: workerPendingTTL,
+			Interval:    workerInterval,
+			Batch:       workerBatch,
+			PendingTTL:  workerPendingTTL,
+			Concurrency: workerConcurrency,
 		},
 		Log: Log{
 			Level:  logLevel,