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>

backend/app/routers/photos.py

@@ -352,6 +352,74 @@ async def list_photos_with_gps(
     ]
 
 
+@router.get("/memories")
+async def get_memories(
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """'On this day' — photos taken on this date in previous years.
+
+    Returns groups keyed by year, each with up to 12 photos. Only
+    considers non-discarded, non-hidden photos with an EXIF-sourced
+    taken_at (no filesystem-guessed dates to avoid false matches).
+    """
+    from sqlalchemy import extract
+
+    today = datetime.now().date()
+
+    result = await db.execute(
+        select(
+            Photo.id,
+            Photo.filename,
+            Photo.taken_at,
+            Photo.thumb_small,
+            Photo.thumb_medium,
+            Photo.media_type,
+            Photo.width,
+            Photo.height,
+            Photo.rating,
+        )
+        .where(
+            Photo.user_id == current_user.id,
+            Photo.is_discarded.is_(False),
+            Photo.is_hidden.is_(False),
+            Photo.taken_at.is_not(None),
+            Photo.taken_at_source == "exif",
+            extract("month", Photo.taken_at) == today.month,
+            extract("day", Photo.taken_at) == today.day,
+            extract("year", Photo.taken_at) < today.year,
+        )
+        .order_by(Photo.taken_at.desc())
+    )
+    rows = result.all()
+
+    # Group by year
+    years: dict[int, list] = {}
+    for row in rows:
+        year = row.taken_at.year
+        group = years.setdefault(year, [])
+        if len(group) >= 12:
+            continue
+        group.append({
+            "id": row.id,
+            "filename": row.filename,
+            "taken_at": row.taken_at.isoformat(),
+            "thumb_small": row.thumb_small,
+            "thumb_medium": row.thumb_medium,
+            "media_type": row.media_type,
+            "width": row.width,
+            "height": row.height,
+            "rating": row.rating,
+        })
+
+    memories = [
+        {"year": year, "years_ago": today.year - year, "photos": photos}
+        for year, photos in sorted(years.items())
+    ]
+
+    return {"date": today.isoformat(), "memories": memories}
+
+
 @router.get("/{photo_id}")
 async def get_photo(
     photo_id: str,