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

shared/schema.ts

@@ -209,6 +209,7 @@ export const musicFavorites = pgTable("music_favorites", {
 export const appConfig = pgTable("app_config", {
   id: varchar("id").primaryKey().default(sql`gen_random_uuid()`),
   externalApiKey: text("external_api_key"),
+  geminiApiKey: text("gemini_api_key"),
   createdAt: timestamp("created_at").defaultNow(),
   updatedAt: timestamp("updated_at").defaultNow(),
 });