feat: safe deletion, album/asset delete, trash, README update

- volume-aware deletion: read-only volumes remove DB only, writable
  volumes soft-delete to trash with configurable grace period
- trash page with restore, worker purge sweep (TRASH_RETENTION_DAYS)
- album delete endpoint + sidebar trash icon
- asset delete from timeline selection toolbar
- all listing queries exclude trashed assets (deleted_at IS NULL)
- timeline ordered by EXIF capture date, date-summary endpoint
- README rewritten with features, setup, full env var table

.env.example

@@ -39,6 +39,11 @@ STORAGE_PATH=./data/media
 # ============================================================================
 # MAX_UPLOAD_BYTES=268435456
 
+# ============================================================================
+# Trash (default 30 days before permanent purge)
+# ============================================================================
+# TRASH_RETENTION_DAYS=30
+
 # ============================================================================
 # Logging
 # ============================================================================