feat(security): implement OAuth2 authorization server with database support

- Add OAuthServer for handling OAuth2 flows including authorization, token exchange, and client registration.
- Introduce DatabaseAuthenticator for persisting clients and authorization codes.
- Implement SQL procedures for client registration, code saving, and token introspection.
- Support for external OAuth2 providers and PKCE (Proof Key for Code Exchange).

pkg/resolvemcp/oauth2_server.go

@@ -0,0 +1,51 @@
+package resolvemcp
+
+import (
+	"net/http"
+
+	"github.com/bitechdev/ResolveSpec/pkg/security"
+)
+
+// EnableOAuthServer activates the MCP-standard OAuth2 authorization server on this Handler.
+//
+// Pass a DatabaseAuthenticator to enable direct username/password login — the server acts as
+// its own identity provider and renders a login form at /oauth/authorize. Pass nil to use
+// only external providers registered via RegisterOAuth2Provider.
+//
+// After calling this, HTTPHandler and StreamableHTTPMux serve the full set of RFC-compliant
+// endpoints required by MCP clients alongside the MCP transport:
+//
+//	GET  /.well-known/oauth-authorization-server   RFC 8414 — auto-discovery
+//	POST /oauth/register                            RFC 7591 — dynamic client registration
+//	GET  /oauth/authorize                           OAuth 2.1 + PKCE — start login
+//	POST /oauth/authorize                           Login form submission (password flow)
+//	POST /oauth/token                               Bearer token exchange + refresh
+//	GET  /oauth/provider/callback                   External provider redirect target
+func (h *Handler) EnableOAuthServer(cfg security.OAuthServerConfig, auth *security.DatabaseAuthenticator) {
+	h.oauthSrv = security.NewOAuthServer(cfg, auth)
+	// Wire any external providers already registered via RegisterOAuth2
+	for _, reg := range h.oauth2Regs {
+		h.oauthSrv.RegisterExternalProvider(reg.auth, reg.cfg.ProviderName)
+	}
+}
+
+// RegisterOAuth2Provider adds an external OAuth2 provider to the MCP OAuth2 authorization server.
+// EnableOAuthServer must be called before this. The auth must have been configured with
+// WithOAuth2(providerName, ...) for the given provider name.
+func (h *Handler) RegisterOAuth2Provider(auth *security.DatabaseAuthenticator, providerName string) {
+	if h.oauthSrv != nil {
+		h.oauthSrv.RegisterExternalProvider(auth, providerName)
+	}
+}
+
+// mountOAuthServerRoutes mounts the security.OAuthServer's HTTP handler onto mux.
+func (h *Handler) mountOAuthServerRoutes(mux *http.ServeMux) {
+	oauthHandler := h.oauthSrv.HTTPHandler()
+	// Delegate all /oauth/ and /.well-known/ paths to the OAuth server
+	mux.Handle("/.well-known/", oauthHandler)
+	mux.Handle("/oauth/", oauthHandler)
+	if h.oauthSrv != nil {
+		// Also mount the external provider callback path if it differs from /oauth/
+		mux.Handle(h.oauthSrv.ProviderCallbackPath(), oauthHandler)
+	}
+}