feat: "On this day" memories — photos from previous years

Add a Memories view that surfaces photos taken on the current date in
previous years (like Google Photos / Immich). Only uses EXIF-sourced
dates to avoid false matches from filesystem timestamps.

- Backend: GET /api/v1/photos/memories returns groups by year, up to
  12 photos each, filtered to non-discarded/non-hidden EXIF dates
- Frontend: MemoriesView with year-grouped thumbnail grid
- Sidebar: new "Memories" nav item with clock icon

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

frontend/src/services/api.ts

@@ -328,6 +328,12 @@ export const photos = {
   getProxyUrl: (photoId: string) => {
     return `${API_BASE_URL}/photos/${photoId}/proxy`
   },
+
+  /** "On this day" memories — photos taken on this date in previous years. */
+  memories: async (): Promise<MemoriesResponse> => {
+    const response = await api.get('/photos/memories')
+    return response.data
+  },
 }
 
 // Library API
@@ -577,6 +583,31 @@ export interface DuplicateGroupsResponse {
   total_members: number
 }
 
+// ── Memories ("On this day") ────────────────────────────────────────────
+
+export interface MemoryPhoto {
+  id: string
+  filename: string
+  taken_at: string
+  thumb_small: string | null
+  thumb_medium: string | null
+  media_type: string
+  width: number | null
+  height: number | null
+  rating: number
+}
+
+export interface MemoryGroup {
+  year: number
+  years_ago: number
+  photos: MemoryPhoto[]
+}
+
+export interface MemoriesResponse {
+  date: string
+  memories: MemoryGroup[]
+}
+
 export interface LibraryStats {
   all_photos: number
   rated: number