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/handlers_auth.go

@@ -15,6 +15,7 @@ import (
 func (u *UIServer) handleLoginPage(w http.ResponseWriter, r *http.Request) {
 	u.render(w, "login", LoginData{
 		WebAuthnEnabled: u.cfg.WebAuthnEnabled(),
+		SSONonce:        r.URL.Query().Get("sso"),
 	})
 }
 
@@ -97,6 +98,8 @@ func (u *UIServer) handleLoginPost(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	ssoNonce := r.FormValue("sso_nonce")
+
 	// TOTP required: issue a server-side nonce and show the TOTP step form.
 	// Security: the nonce replaces the password hidden field (F-02). The password
 	// is not stored anywhere after this point; only the account ID is retained.
@@ -110,11 +113,12 @@ func (u *UIServer) handleLoginPost(w http.ResponseWriter, r *http.Request) {
 		u.render(w, "totp_step", LoginData{
 			Username: username,
 			Nonce:    nonce,
+			SSONonce: ssoNonce,
 		})
 		return
 	}
 
-	u.finishLogin(w, r, acct)
+	u.finishLogin(w, r, acct, ssoNonce)
 }
 
 // handleTOTPStep handles the second POST when totp_step=1 is set.
@@ -129,6 +133,7 @@ func (u *UIServer) handleTOTPStep(w http.ResponseWriter, r *http.Request) {
 	username := r.FormValue("username")  //nolint:gosec // body already limited by caller
 	nonce := r.FormValue("totp_nonce")   //nolint:gosec // body already limited by caller
 	totpCode := r.FormValue("totp_code") //nolint:gosec // body already limited by caller
+	ssoNonce := r.FormValue("sso_nonce") //nolint:gosec // body already limited by caller
 
 	// Security: consume the nonce (single-use); reject if unknown or expired.
 	accountID, ok := u.consumeTOTPNonce(nonce)
@@ -172,6 +177,7 @@ func (u *UIServer) handleTOTPStep(w http.ResponseWriter, r *http.Request) {
 			Error:    "invalid TOTP code",
 			Username: username,
 			Nonce:    newNonce,
+			SSONonce: ssoNonce,
 		})
 		return
 	}
@@ -189,15 +195,28 @@ func (u *UIServer) handleTOTPStep(w http.ResponseWriter, r *http.Request) {
 			Error:    "invalid TOTP code",
 			Username: username,
 			Nonce:    newNonce,
+			SSONonce: ssoNonce,
 		})
 		return
 	}
 
-	u.finishLogin(w, r, acct)
+	u.finishLogin(w, r, acct, ssoNonce)
 }
 
 // finishLogin issues a JWT, sets the session cookie, and redirects to dashboard.
-func (u *UIServer) finishLogin(w http.ResponseWriter, r *http.Request, acct *model.Account) {
+// When ssoNonce is non-empty, the login is part of an SSO redirect flow: instead
+// of setting a session cookie, an authorization code is issued and the user is
+// redirected back to the service's callback URL.
+func (u *UIServer) finishLogin(w http.ResponseWriter, r *http.Request, acct *model.Account, ssoNonce string) {
+	// SSO redirect flow: issue authorization code and redirect to service.
+	if ssoNonce != "" {
+		if callbackURL, ok := u.buildSSOCallback(r, ssoNonce, acct.ID); ok {
+			http.Redirect(w, r, callbackURL, http.StatusFound)
+			return
+		}
+		// SSO session expired/consumed — fall through to normal login.
+	}
+
 	// Determine token expiry based on admin role.
 	expiry := u.cfg.DefaultExpiry()
 	roles, err := u.db.GetRoles(acct.ID)