docs(config): add config.example.yml with correct top-level gemini/jwt/smtp

[Bhup-GitHUB]

Summary

Add backend/config/config.example.yml as the canonical configuration template.
Ensure structure matches config.Config in backend/config/config.go:
gemini, jwt, and smtp should be top-level keys (siblings of server, database, and googleOAuth).

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Why

The previous example incorrectly nested these keys under database, causing cfg.Gemini.ApiKey to be empty and resulting in the error:

“Failed to initialize Gemini client: You need an auth option to use this client.”

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Details

• Adds placeholders for all secrets — no real credentials.
• Keeps database.uri unchanged (placeholder only).
• Aligns YAML structure with Go config parsing logic.

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How to Verify

1. Populate the secrets in config.example.yml.
2. Run the backend.
3. The Gemini client should successfully initialize using cfg.Gemini.ApiKey.

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Notes

Consider adding backend/config/config.prod.yml to .gitignore to prevent committing secrets in local setups.

[Bhup-GitHUB]

@rixitgithub
The PR fixes the config structure issue and provides a proper example template for easier backend initialization, especially for new contributors.