fix: high-severity findings from code audit

- backend/photos: whitelist sortable columns instead of getattr(Photo, sort).
  Previously any client-supplied string was passed to SQLAlchemy, exposing
  every Photo attribute (filepath, file_hash, etc.) as a sort target.
- App: move the auto-show-right-sidebar logic out of the render body and
  into a useEffect. The previous version called setState during render,
  causing extra re-render passes the audit caught.
- types/photo: add added_at and tighten folder_id from optional to nullable.
  Drops a (photo as any).added_at cast in Timeline.
- constants/colorLabels: extract a single COLOR_LABEL_OPTIONS used by
  FilterBar, RightSidebar, and PhotoInfoPanel. filterStore re-exports the
  ColorLabel type so existing imports keep working.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/store/filterStore.ts

@@ -1,7 +1,8 @@
 import { create } from 'zustand'
+import type { ColorLabel } from '../constants/colorLabels'
 
 export type MediaType = 'photo' | 'video' | 'raw' | 'heic'
-export type ColorLabel = 'red' | 'orange' | 'yellow' | 'green' | 'blue' | 'purple'
+export type { ColorLabel }
 export type FlagFilter = 'any' | 'discarded'
 export type SortField =
   | 'taken_at'