Add photo slideshow screensaver with inactivity timer

- After 1 hour of inactivity the screen switches to a fullscreen
  crossfade slideshow of photos from static/photos/
- Touch anywhere on screen to dismiss and return to the calendar
- /api/photos endpoint serves a shuffled list of images from static/photos/
- Two-slot A/B image swap for smooth 1.5s crossfades; next image preloads
  in the background while current is displayed (15s per photo)
- Slideshow only activates if static/photos/ contains at least one image

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app.py

@@ -235,6 +235,20 @@ def get_calendar():
         return jsonify(empty_week_payload(week_start_date, members_for_filter))
 
 
+@app.route('/api/photos')
+def get_photos():
+    """Return a shuffled list of all image URLs from the photos directory."""
+    photos_dir = app.config['PHOTOS_DIR']
+    if not os.path.exists(photos_dir):
+        return jsonify([])
+    images = [
+        f'/static/photos/{f}' for f in os.listdir(photos_dir)
+        if f.lower().endswith(('.png', '.jpg', '.jpeg', '.gif', '.webp'))
+    ]
+    random.shuffle(images)
+    return jsonify(images)
+
+
 @app.route('/api/background')
 def get_background():
     """Get a random background image."""
@@ -289,8 +303,8 @@ def get_joke():
 
 
 if __name__ == '__main__':
-    # Create backgrounds directory if it doesn't exist
     os.makedirs(app.config['BACKGROUNDS_DIR'], exist_ok=True)
+    os.makedirs(app.config['PHOTOS_DIR'], exist_ok=True)
     os.makedirs(app.config['CREDENTIALS_DIR'], exist_ok=True)
 
     # Run the app