Add Google Identity-Aware Proxy (IAP) support

[sharoonthomas]

Add support for Google Cloud Identity-Aware Proxy to automatically identify and track users creating pastas when deployed behind Google IAP.

Changes

• Add MICROBIN_ENABLE_GOOGLE_IAP environment variable to enable/disable IAP support
• Add author field to Pasta struct to store user identification
• Implement extract_google_iap_user() function to parse IAP headers
• Update create endpoint to extract and store author information from request headers
• Update database schemas (SQLite and JSON) to include author field
• Add author display in pasta view and list templates
• Maintain backward compatibility - existing pastas without author info work normally

Technical Details

Google IAP sends user information in HTTP headers:

• X-Goog-Authenticated-User-Email: "accounts.google.com:user@example.com"
• X-Goog-Authenticated-User-Id: "accounts.google.com:123456789"

The implementation prioritizes email addresses over user IDs for better
user experience. When IAP is disabled, the feature is completely inactive.

Usage

Local Development

# Test with simulated IAP headers
MICROBIN_ENABLE_GOOGLE_IAP=true cargo run

# Test with curl
curl -X POST http://localhost:8080/upload \
  -H "X-Goog-Authenticated-User-Email: accounts.google.com:test@example.com" \
  -F "content=Hello World"

Production Deployment

1. Deploy to Google Cloud Run (or other services) with Identity-Aware Proxy enabled
2. Set environment variable: MICROBIN_ENABLE_GOOGLE_IAP=true
3. Users will be automatically identified and their email/ID stored with each pasta

Benefits

• User Accountability: Track who created each pasta for audit purposes
• Enterprise Ready: Integrates with Google Workspace authentication
• Zero Configuration: Works automatically when IAP is enabled
• Backward Compatible: Existing installations continue to work unchanged
• Privacy Conscious: Only stores user info when explicitly enabled

Database Migration

Existing databases will automatically handle the new author field:

• SQLite: New column added as nullable TEXT
• JSON: Existing entries will have author: null

No manual migration required for existing installations.

[sharoonthomas]

@szabodanika Would love some feedback on the implementation and if this makes sense?

I could not find where the docs are either.