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/main.py

@@ -24,6 +24,8 @@ templates = Jinja2Templates(directory="app/templates")
 
 MIN_REFRESH_INTERVAL_S = 60
 MAX_REFRESH_INTERVAL_S = 86400
+MIN_QUEUE_TARGET_LEN = 5
+MAX_QUEUE_TARGET_LEN = 50
 
 
 @app.get("/health")
@@ -65,6 +67,7 @@ def api_config_save(
     order: str = Form("sequential"),
     refresh_interval_s: int = Form(3600),
     smart_crop_faces: bool = Form(True),
+    queue_target_len: int = Form(20),
 ):
     # Immich URL/API key are env-var only (IMMICH_URL/IMMICH_API_KEY, see
     # docker-compose.yml.example) -- config.load() already applies them,
@@ -82,6 +85,7 @@ def api_config_save(
     cfg.order = order if order in ("sequential", "shuffle") else "sequential"
     cfg.refresh_interval_s = max(MIN_REFRESH_INTERVAL_S, min(MAX_REFRESH_INTERVAL_S, refresh_interval_s))
     cfg.smart_crop_faces = smart_crop_faces
+    cfg.queue_target_len = max(MIN_QUEUE_TARGET_LEN, min(MAX_QUEUE_TARGET_LEN, queue_target_len))
     config.save(cfg)
     return {"status": "saved"}
 
@@ -166,7 +170,10 @@ def api_queue():
     client = ImmichClient(cfg.immich_url, cfg.immich_api_key)
     assets = _list_assets(client, cfg)
 
-    if photo_queue.get_current(cfg, assets):
+    current_changed = photo_queue.get_current(cfg, assets)
+    queue_before = list(cfg.queue)
+    photo_queue.sync_queue_length(cfg, assets)
+    if current_changed or cfg.queue != queue_before:
         config.save(cfg)
 
     def entry(asset_id: str) -> dict: