chore: Add .secrets to .gitignore and configure internal sync secret #5

- Add .secrets file to .gitignore to prevent credential leakage
- Generate secure INTERNAL_SYNC_SECRET for scheduled sync authentication
- Add ShopRenter OAuth placeholders for future configuration
- Store Supabase credentials and project info in .secrets file

.gitignore

@@ -7,6 +7,7 @@ bun.lockb
 .env
 .env.*
 !.env.example
+.secrets
 
 # Build output
 dist/