feat: move voice enable/model config to admin UI

Replace VOICE_ENABLED env var gate with DB-backed admin setting.

- services/voice_config.py: reads voice_enabled + voice_stt_model from
  admin user's settings row (falls back to env var defaults)
- routes/admin.py: GET/PUT /api/admin/voice for admin configuration
- routes/voice.py, services/stt.py, services/tts.py: read enabled/model
  from DB via voice_config instead of Config directly
- app.py: always schedule model loaders at startup; they self-gate on
  the DB setting so no conditional needed at the call site
- SettingsView.vue: Voice section in Admin → Config tab (enable toggle +
  STT model dropdown); user Voice tab now points to admin panel when disabled

No env var required to test — enable via Settings → Admin → Config → Voice.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/app.py

@@ -266,12 +266,11 @@ def create_app() -> Quart:
         from fabledassistant.services.briefing_scheduler import start_briefing_scheduler
         await start_briefing_scheduler(asyncio.get_running_loop())
 
-        # Voice model loading (only when feature is enabled)
-        if Config.VOICE_ENABLED:
-            from fabledassistant.services.stt import load_stt_model
-            from fabledassistant.services.tts import load_tts_model
-            asyncio.create_task(load_stt_model())
-            asyncio.create_task(load_tts_model())
+        # Voice model loading (enabled via Admin → Config in the UI, or VOICE_ENABLED env var)
+        from fabledassistant.services.stt import load_stt_model
+        from fabledassistant.services.tts import load_tts_model
+        asyncio.create_task(load_stt_model())
+        asyncio.create_task(load_tts_model())
 
     @app.after_serving
     async def shutdown():