Add Google Gemini TTS as alternative to Edge TTS

- Store Gemini API key globally in appConfig (single key for all users)
- Add /api/settings/gemini GET/POST/DELETE routes for API key management
- Backend: add tts_engine parameter ("edge" or "gemini") to FFmpeg service
- Backend: add generate_tts_gemini() using Google Cloud TTS REST API
- Frontend: Settings page shows Google Gemini API key input card
- Frontend: new-reel, mobile/new-reel, remotion-video, mobile/remotion-video
  pages now have Edge/Gemini engine toggle and French voice selector
- Fix tts-preview route to extract ttsVoice from req.body instead of
  undefined voice variable

ffmpeg-service/requirements.txt

@@ -6,3 +6,4 @@ pydantic==2.6.0
 edge-tts==6.1.12
 emoji
 ffsubsync==0.4.26
+httpx>=0.25.0