Stop leaking the Gemini API key through /gemini_key

The settings endpoint returned the raw key to any caller, and the
frontend used that response to pre-fill the input field, which also
meant saving with an untouched field would silently resubmit the
plaintext key. /gemini_key now reports only whether a key is set and
a masked preview; the UI shows that as a placeholder and only updates
the stored key when the field is actually filled in.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

server.py

@@ -1248,7 +1248,8 @@ async def set_epg_url(request):
 async def get_gemini_key(request):
     doc = settings_collection.find_one({'key': 'gemini_key'})
     key = doc.get('value', '') if doc else ''
-    return web.json_response({'gemini_key': key})
+    masked_key = (key[:4] + '...' + key[-4:]) if len(key) > 8 else ('****' if key else '')
+    return web.json_response({'configured': bool(key), 'masked_key': masked_key})
 
 
 async def set_gemini_key(request):