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/App.tsx

@@ -2,6 +2,7 @@ import { useState } from 'react'
 import { Timeline } from './components/timeline/Timeline'
 import { DuplicatesView } from './components/duplicates/DuplicatesView'
 import { MapView } from './components/map/MapView'
+import { MemoriesView } from './components/memories/MemoriesView'
 import { PeopleView } from './components/people/PeopleView'
 import { TagsView } from './components/tags/TagsView'
 import { ColorsView } from './components/colors/ColorsView'
@@ -87,6 +88,8 @@ function MainApp() {
               <SettingsPage />
             ) : currentSection === 'map' ? (
               <MapView />
+            ) : currentSection === 'memories' ? (
+              <MemoriesView />
             ) : currentSection === 'duplicates' ? (
               <DuplicatesView />
             ) : currentSection === 'people' ? (