Add SSO authorization code flow (Phase 1)

MCIAS now acts as an SSO provider for downstream services. Services
redirect users to /sso/authorize, MCIAS handles login (password, TOTP,
or passkey), then redirects back with an authorization code that the
service exchanges for a JWT via POST /v1/sso/token.

- Add SSO client registry to config (client_id, redirect_uri,
  service_name, tags) with validation
- Add internal/sso package: authorization code and session stores
  using sync.Map with TTL, single-use LoadAndDelete, cleanup goroutines
- Add GET /sso/authorize endpoint (validates client, creates session,
  redirects to /login?sso=<nonce>)
- Add POST /v1/sso/token endpoint (exchanges code for JWT with policy
  evaluation using client's service_name/tags from config)
- Thread SSO nonce through password→TOTP and WebAuthn login flows
- Update login.html, totp_step.html, and webauthn.js for SSO nonce
  passthrough

Security:
- Authorization codes are 256-bit random, single-use, 60-second TTL
- redirect_uri validated as exact match against registered config
- Policy context comes from MCIAS config, not the calling service
- SSO sessions are server-side only; nonce is the sole client-visible value
- WebAuthn SSO returns redirect URL as JSON (not HTTP redirect) for JS compat

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/ui/ui.go

@@ -445,6 +445,9 @@ func (u *UIServer) Register(mux *http.ServeMux) {
 	uiMux.HandleFunc("GET /unseal", u.handleUnsealPage)
 	uiMux.Handle("POST /unseal", unsealRateLimit(http.HandlerFunc(u.handleUnsealPost)))
 
+	// SSO authorize route (no session required, rate-limited).
+	uiMux.Handle("GET /sso/authorize", loginRateLimit(http.HandlerFunc(u.handleSSOAuthorize)))
+
 	// Auth routes (no session required).
 	uiMux.HandleFunc("GET /login", u.handleLoginPage)
 	uiMux.Handle("POST /login", loginRateLimit(http.HandlerFunc(u.handleLoginPost)))
@@ -810,6 +813,7 @@ type PageData struct {
 type LoginData struct {
 	Error    string
 	Username string // pre-filled on TOTP step
+	SSONonce string // SSO session nonce (hidden field for SSO redirect flow)
 	// Security (F-02): Password is no longer carried in the HTML form. Instead
 	// a short-lived server-side nonce is issued after successful password
 	// verification, and only the nonce is embedded in the TOTP step form.