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/voice_config.py

@@ -0,0 +1,40 @@
+"""System-wide voice configuration stored in the admin user's settings row.
+
+Falls back to Config env vars so operators can still seed defaults via the
+environment without touching the UI.
+"""
+from sqlalchemy import select
+
+from fabledassistant.config import Config
+from fabledassistant.models import async_session
+from fabledassistant.models.setting import Setting
+from fabledassistant.models.user import User
+
+VOICE_SETTING_KEYS = ("voice_enabled", "voice_stt_model")
+
+
+async def get_voice_config() -> dict[str, str]:
+    """Return voice config, preferring DB values over env-var defaults."""
+    config: dict[str, str] = {
+        "voice_enabled": "true" if Config.VOICE_ENABLED else "false",
+        "voice_stt_model": Config.STT_MODEL,
+    }
+    async with async_session() as session:
+        result = await session.execute(
+            select(Setting)
+            .join(User, Setting.user_id == User.id)
+            .where(User.role == "admin", Setting.key.in_(VOICE_SETTING_KEYS))
+        )
+        for setting in result.scalars().all():
+            config[setting.key] = setting.value
+    return config
+
+
+async def is_voice_enabled() -> bool:
+    config = await get_voice_config()
+    return config.get("voice_enabled", "false").lower() in ("1", "true", "yes")
+
+
+async def get_stt_model() -> str:
+    config = await get_voice_config()
+    return config.get("voice_stt_model", Config.STT_MODEL)