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/photo_queue.py

@@ -8,9 +8,10 @@ photo instead of silently skipping ahead) or an explicit forced advance
 button -- ignoring elapsed time).
 
 `queue` is a small reorderable lookahead the web UI can preview and
-rearrange, topped up automatically from the album as it's consumed.
-`queue_cursor` is separate, internal-only bookkeeping for where sequential
-top-up resumes in the album -- not shown or reordered in the UI.
+rearrange, topped up (or trimmed) automatically to match
+`cfg.queue_target_len` (user-configurable from the web UI) as it's
+consumed. `queue_cursor` is separate, internal-only bookkeeping for where
+sequential top-up resumes in the album -- not shown or reordered in the UI.
 """
 
 from __future__ import annotations
@@ -20,14 +21,20 @@ import time
 
 from .config import FrameConfig
 
-QUEUE_TARGET_LEN = 24
-
 
 def _top_up(cfg: FrameConfig, assets: list[dict]) -> None:
     valid_ids = {a["id"] for a in assets}
     cfg.queue = [asset_id for asset_id in cfg.queue if asset_id in valid_ids]
 
-    needed = QUEUE_TARGET_LEN - len(cfg.queue)
+    target = cfg.queue_target_len
+    if len(cfg.queue) > target:
+        # Target was lowered since this queue was built -- shrink it
+        # immediately rather than waiting for enough advances to consume
+        # the excess naturally.
+        cfg.queue = cfg.queue[:target]
+        return
+
+    needed = target - len(cfg.queue)
     if needed <= 0 or not assets:
         return
 
@@ -72,13 +79,21 @@ def advance_forced(cfg: FrameConfig, assets: list[dict]) -> None:
         # only asset is already current) -- keep showing what we have.
         cfg.current_asset_id = assets[0]["id"]
     cfg.current_asset_set_at = time.time()
-    # Refill back up to QUEUE_TARGET_LEN now that current_asset_id has
+    # Refill back up to queue_target_len now that current_asset_id has
     # changed -- otherwise the queue is left one short until the *next*
     # advance, since the pop above consumes one of the items _top_up just
     # added.
     _top_up(cfg, assets)
 
 
+def sync_queue_length(cfg: FrameConfig, assets: list[dict]) -> None:
+    """Tops up or trims cfg.queue to match cfg.queue_target_len without
+    otherwise touching current_asset_id. Used by GET /api/queue so a
+    change to the "upcoming photos to show" setting takes effect on page
+    load rather than waiting for the next natural advance."""
+    _top_up(cfg, assets)
+
+
 def get_current(cfg: FrameConfig, assets: list[dict]) -> bool:
     """Time-based, idempotent path used by GET /frame/image. Advances only
     if the current photo is unset/invalid or refresh_interval_s has