refactor: remove Piper TTS, keep edge_tts only

- Remove piper_url from ReelRequest model and all function signatures
- Remove generate_tts_piper() function and Piper branch in generate_tts_with_subs()
- Remove /api/piper/config routes from server/routes.ts
- Remove piperConfig table, schemas and storage methods
- Remove piper_config migration
- Remove Piper TTS settings UI card
- Update "Piper TTS" labels to "TTS — voix activée"

edge_tts is now the only TTS engine, using precise word-boundary timing

client/src/pages/mobile/remotion-video.tsx

@@ -311,7 +311,7 @@ export default function MobileRemotionVideoPage() {
             </div>
             {ttsEnabled && (
               <div className="space-y-2">
-                <p className="text-xs text-muted-foreground">Piper TTS — voix configurée sur le serveur</p>
+                <p className="text-xs text-muted-foreground">TTS — voix activée</p>
                 <Button size="sm" variant="outline" className="w-full" onClick={handleTtsPreview} disabled={!overlayText}>
                   <Volume2 className="mr-2 w-3 h-3" /> Écouter la voix
                 </Button>