Scope thumbnail access to the frame's own photos; validate Gitea repo URL

Two fixes from a security pass over the server:

- /api/frames/{id}/thumbnail/{asset_id} accepted any asset id and
  fetched it via the frame owner's Immich credentials, unscoped to what
  that frame actually shows -- a user merely linked to view a frame
  could pull thumbnails for any asset in the owner's whole library, not
  just the frame's own album. Now scoped to current_asset_id/queue,
  matching the check device.frame_share and manage.manage_thumbnail
  already both apply.

- firmware_update_repo_url now has to be a plain http(s) URL. Unlike a
  one-off manual firmware upload (a deliberate, explicit act -- left
  alone), auto-update from a repo is a standing trust relationship: the
  frame keeps fetching from it and, with auto-update on, installs
  whatever it finds with nobody reviewing it first. Added a plain-
  language note next to the checkbox saying exactly that.

server/app/routers/api_frames.py

@@ -17,6 +17,7 @@ from __future__ import annotations
 
 import logging
 import time
+from urllib.parse import urlparse
 
 import httpx
 from fastapi import APIRouter, Depends, File, Form, HTTPException, Request, UploadFile
@@ -59,6 +60,17 @@ MAX_QUEUE_TARGET_LEN = 5000
 ORIENTATIONS = ("landscape", "portrait", "landscape_flipped", "portrait_flipped")
 
 
+def _valid_repo_url(url: str) -> bool:
+    """The frame will periodically fetch from this URL on its own (see
+    gitea_releases.py) and, with auto-update on, install whatever it
+    finds -- unlike a one-off manual firmware upload, that's a standing
+    trust relationship, so it's worth rejecting obviously-wrong input at
+    save time rather than only failing later at fetch time. http(s) only
+    -- no file://, no other schemes."""
+    parsed = urlparse(url)
+    return parsed.scheme in ("http", "https") and bool(parsed.netloc)
+
+
 @router.get("/api/frames/{frame_id}/albums")
 def api_albums(frame: Frame = Depends(require_frame_view)):
     url, key = immich_creds(frame)
@@ -129,7 +141,10 @@ def api_config_save(
         if timezone is not None and timezone in quiet_hours.ALL_TIMEZONES:
             cfg.timezone = timezone
         if firmware_update_repo_url is not None:
-            cfg.firmware_update_repo_url = firmware_update_repo_url.strip()
+            stripped = firmware_update_repo_url.strip()
+            if stripped and not _valid_repo_url(stripped):
+                raise HTTPException(400, "Firmware repo URL must be a plain http:// or https:// URL")
+            cfg.firmware_update_repo_url = stripped
         if firmware_auto_update is not None:
             cfg.firmware_auto_update = firmware_auto_update
         if battery_alert_threshold_pct is not None:
@@ -321,7 +336,14 @@ def api_queue_remove(
 
 @router.get("/api/frames/{frame_id}/thumbnail/{asset_id}")
 def api_thumbnail(asset_id: str, frame: Frame = Depends(require_frame_view)):
+    """Scoped to what this frame is actually showing/queuing -- a user
+    merely linked to view this frame shouldn't be able to pull thumbnails
+    for arbitrary asset ids in the owner's Immich library, only the
+    frame's own curated album. Same rule device.frame_share and
+    manage.manage_thumbnail already enforce."""
     require_configured(frame)
+    if asset_id != frame.current_asset_id and asset_id not in frame.queue:
+        raise HTTPException(404, "Not on this frame")
     client = immich_client_for(frame)
     try:
         content, content_type = client.download_asset_thumbnail(asset_id)

server/app/templates/frame_config.html

@@ -106,6 +106,9 @@
           <input type="checkbox" id="firmware_auto_update" {% if frame.firmware_auto_update %}checked{% endif %}>
           <label for="firmware_auto_update">Automatically apply updates</label>
         </div>
+        <p class="sub" style="margin-top: 4px;">While on, this frame installs
+          whatever the repo above publishes next, with nobody reviewing it
+          first -- only point it at a repo you trust.</p>
         <button type="button" class="secondary" id="firmware-settings-save">Save</button>
         <p class="sub" id="firmware-gitea-status" style="display: none; margin-top: 10px;"></p>
         <button type="button" class="secondary" id="firmware-check-now">Check now</button>