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/components/layout/LeftSidebar.tsx

@@ -23,6 +23,7 @@ import {
   User as UserIcon,
   LogOut,
   Shield,
+  Clock,
 } from 'lucide-react'
 import clsx from 'clsx'
 import { sourceFolders, photos as photosApi, type FolderTreeNode } from '../../services/api'
@@ -407,6 +408,7 @@ export function LeftSidebar({ onCollapse }: LeftSidebarProps) {
         { id: 'people', label: 'People', icon: <Users className="h-4 w-4" />, count: peopleTotalCount },
         { id: 'colors', label: 'Colors', icon: <Palette className="h-4 w-4" />, count: stats?.colored ?? 0 },
         { id: 'map', label: 'Map', icon: <MapPin className="h-4 w-4" />, count: stats?.with_gps ?? 0 },
+        { id: 'memories', label: 'Memories', icon: <Clock className="h-4 w-4" /> },
         { id: 'duplicates', label: 'Duplicates', icon: <Copy className="h-4 w-4" />, count: stats?.duplicates ?? 0 },
         { id: 'discarded', label: 'Discarded', icon: <Trash2 className="h-4 w-4" />, count: stats?.discarded ?? 0 },
       ],

frontend/src/components/memories/MemoriesView.tsx

@@ -0,0 +1,70 @@
+import { useQuery } from '@tanstack/react-query'
+import { photos } from '../../services/api'
+import type { MemoryGroup } from '../../services/api'
+
+export function MemoriesView() {
+  const { data, isLoading } = useQuery({
+    queryKey: ['memories'],
+    queryFn: () => photos.memories(),
+    staleTime: 60_000 * 30, // 30 min — date doesn't change often
+  })
+
+  if (isLoading) {
+    return (
+      <div className="flex items-center justify-center h-64 text-neutral-500">
+        Loading memories...
+      </div>
+    )
+  }
+
+  const memories = data?.memories ?? []
+
+  if (memories.length === 0) {
+    return (
+      <div className="flex flex-col items-center justify-center h-64 text-neutral-500 gap-2">
+        <p className="text-lg font-medium">No memories for today</p>
+        <p className="text-sm">Photos taken on this date in previous years will appear here.</p>
+      </div>
+    )
+  }
+
+  return (
+    <div className="p-6 space-y-8 max-w-5xl mx-auto">
+      <h2 className="text-xl font-semibold text-neutral-200">
+        On This Day &mdash; {data?.date}
+      </h2>
+
+      {memories.map((group: MemoryGroup) => (
+        <section key={group.year} className="space-y-3">
+          <h3 className="text-sm font-medium text-neutral-400 uppercase tracking-wide">
+            {group.year} &middot; {group.years_ago} year{group.years_ago !== 1 ? 's' : ''} ago
+          </h3>
+          <div className="grid grid-cols-2 sm:grid-cols-3 md:grid-cols-4 lg:grid-cols-6 gap-2">
+            {group.photos.map((photo) => (
+              <div
+                key={photo.id}
+                className="aspect-square rounded-lg overflow-hidden bg-neutral-800 relative group"
+              >
+                {photo.thumb_small ? (
+                  <img
+                    src={`/api/v1/photos/${photo.id}/thumb/small`}
+                    alt={photo.filename}
+                    className="w-full h-full object-cover"
+                    loading="lazy"
+                  />
+                ) : (
+                  <div className="w-full h-full flex items-center justify-center text-neutral-600 text-xs">
+                    No thumb
+                  </div>
+                )}
+                <div className="absolute bottom-0 left-0 right-0 bg-gradient-to-t from-black/60 to-transparent px-2 py-1 opacity-0 group-hover:opacity-100 transition-opacity">
+                  <p className="text-xs text-white truncate">{photo.filename}</p>
+                </div>
+              </div>
+            ))}
+          </div>
+        </section>
+      ))}
+    </div>
+  )
+}