Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

- Add MARKDOWNHUB_BASE_URL to fly.toml for public HTTPS client_id URL
- Fix migration system to track applied migrations (schema_migrations table)
- Add .gitignore (db files, binary)
- Update README: use 127.0.0.1 instead of localhost (RFC 8252 compliance)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Add ATProto OAuth handlers (ClientMetadata, ATProtoLoginPage,
ATProtoLoginSubmit, ATProtoCallback) with full PAR + PKCE + DPoP
support; remove defunct OAuthGitHub/OAuthGoogle handlers and update
server routes accordingly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>