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/services/tts.py

@@ -31,11 +31,11 @@ _VOICES: list[dict] = [
 
 
 async def load_tts_model() -> None:
-    """Load the Kokoro pipeline. Called once at startup when VOICE_ENABLED=true."""
+    """Load the Kokoro pipeline. Called once at startup when voice is enabled."""
     global _pipeline, _load_error
-    from fabledassistant.config import Config
+    from fabledassistant.services.voice_config import is_voice_enabled
 
-    if not Config.VOICE_ENABLED:
+    if not await is_voice_enabled():
         return
 
     async with _pipeline_lock: