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/stt.py

@@ -16,11 +16,11 @@ _load_error: str | None = None
 
 
 async def load_stt_model() -> None:
-    """Load the Whisper model. Called once at startup when VOICE_ENABLED=true."""
+    """Load the Whisper model. Called once at startup when voice is enabled."""
     global _model, _load_error
-    from fabledassistant.config import Config
+    from fabledassistant.services.voice_config import get_stt_model, is_voice_enabled
 
-    if not Config.VOICE_ENABLED:
+    if not await is_voice_enabled():
         return
 
     async with _model_lock:
@@ -29,15 +29,16 @@ async def load_stt_model() -> None:
         try:
             from faster_whisper import WhisperModel
 
-            logger.info("Loading Whisper STT model '%s'...", Config.STT_MODEL)
+            model_name = await get_stt_model()
+            logger.info("Loading Whisper STT model '%s'...", model_name)
             loop = asyncio.get_running_loop()
             _model = await loop.run_in_executor(
                 None,
-                lambda: WhisperModel(Config.STT_MODEL, device="cpu", compute_type="int8"),
+                lambda: WhisperModel(model_name, device="cpu", compute_type="int8"),
             )
-            logger.info("Whisper STT model '%s' loaded", Config.STT_MODEL)
+            logger.info("Whisper STT model '%s' loaded", model_name)
         except Exception:
-            _load_error = f"Failed to load Whisper model '{Config.STT_MODEL}'"
+            _load_error = "Failed to load Whisper STT model"
             logger.exception(_load_error)