Add OAuth 2.1 support for MCP server

[kaospr]

Summary

• Adds OAuth 2.1 authorization for MCP endpoints using Doorkeeper with PKCE (S256 only), authorization code flow, and hashed token storage
• Implements RFC 7591 dynamic client registration at /oauth/register with IP-based rate limiting, redirect URI allowlisting (localhost + claude.ai), and input validation
• Adds Rack middleware (McpOauthMiddleware) that authenticates MCP requests via Bearer tokens and passes signed user identity to MCP tools using MessageVerifier
• Includes .well-known/oauth-protected-resource and .well-known/oauth-authorization-server metadata endpoints
• Scheduled cleanup jobs for expired tokens (30+ days) and stale unused registrations (24h)
• Rack::Attack rate limiting on the registration endpoint

Test plan

• Dynamic registration controller tests (valid params, defaults, validation, IP rate limit, redirect URI restrictions)
• Full PKCE authorization code flow integration test
• Token validation tests (expired, revoked, wrong scope, forged headers)
• Metadata endpoint tests
• Cleanup job tests (expired tokens, stale applications, retention of active records)
• Manual test: register client, complete OAuth flow, use MCP tools with token
• Verify Rack::Attack rate limiting in development

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