Make the upcoming-photos queue length user-configurable

Adds "Upcoming photos to show" to the config UI (queue_target_len, 5-50,
default 20, replacing the hardcoded QUEUE_TARGET_LEN constant). Lowering
it trims the queue immediately on next page load rather than waiting for
enough advances to consume the excess naturally; raising it tops back up
the same way, via a new photo_queue.sync_queue_length() called from
GET /api/queue.

server/app/templates/index.html

@@ -76,6 +76,13 @@
         <input type="checkbox" id="smart_crop_faces" {% if cfg.smart_crop_faces %}checked{% endif %}>
         <label for="smart_crop_faces">Center faces in crop</label>
       </div>
+      <label>Upcoming photos to show
+        <select id="queue_target_len">
+          {% for n in [5, 10, 15, 20, 25, 30, 40, 50] %}
+            <option value="{{ n }}" {% if cfg.queue_target_len == n %}selected{% endif %}>{{ n }}</option>
+          {% endfor %}
+        </select>
+      </label>
       <button type="button" class="secondary" id="load-albums">Load Albums</button>
       <button type="submit">Save</button>
     </form>
@@ -103,6 +110,7 @@
         order: document.getElementById('order').value,
         refresh_interval_s: String(minutes * 60),
         smart_crop_faces: String(document.getElementById('smart_crop_faces').checked),
+        queue_target_len: document.getElementById('queue_target_len').value,
       });
       const resp = await fetch('/api/config', {
         method: 'POST',