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/sso/store_test.go

@@ -0,0 +1,132 @@
+package sso
+
+import (
+	"testing"
+	"time"
+)
+
+func TestStoreAndConsume(t *testing.T) {
+	code, err := Store("mcr", "https://mcr.example.com/cb", "state123", 42)
+	if err != nil {
+		t.Fatalf("Store: %v", err)
+	}
+	if code == "" {
+		t.Fatal("Store returned empty code")
+	}
+
+	ac, ok := Consume(code)
+	if !ok {
+		t.Fatal("Consume returned false for valid code")
+	}
+	if ac.ClientID != "mcr" {
+		t.Errorf("ClientID = %q, want %q", ac.ClientID, "mcr")
+	}
+	if ac.RedirectURI != "https://mcr.example.com/cb" {
+		t.Errorf("RedirectURI = %q", ac.RedirectURI)
+	}
+	if ac.State != "state123" {
+		t.Errorf("State = %q", ac.State)
+	}
+	if ac.AccountID != 42 {
+		t.Errorf("AccountID = %d, want 42", ac.AccountID)
+	}
+}
+
+func TestConsumeSingleUse(t *testing.T) {
+	code, err := Store("mcr", "https://mcr.example.com/cb", "s", 1)
+	if err != nil {
+		t.Fatalf("Store: %v", err)
+	}
+
+	if _, ok := Consume(code); !ok {
+		t.Fatal("first Consume should succeed")
+	}
+	if _, ok := Consume(code); ok {
+		t.Error("second Consume should fail (single-use)")
+	}
+}
+
+func TestConsumeUnknownCode(t *testing.T) {
+	if _, ok := Consume("nonexistent"); ok {
+		t.Error("Consume should fail for unknown code")
+	}
+}
+
+func TestConsumeExpiredCode(t *testing.T) {
+	code, err := Store("mcr", "https://mcr.example.com/cb", "s", 1)
+	if err != nil {
+		t.Fatalf("Store: %v", err)
+	}
+
+	// Manually expire the code.
+	v, loaded := pendingCodes.Load(code)
+	if !loaded {
+		t.Fatal("code not found in pendingCodes")
+	}
+	ac, ok := v.(*AuthCode)
+	if !ok {
+		t.Fatal("unexpected type in pendingCodes")
+	}
+	ac.ExpiresAt = time.Now().Add(-1 * time.Second)
+
+	if _, ok := Consume(code); ok {
+		t.Error("Consume should fail for expired code")
+	}
+}
+
+func TestStoreSessionAndConsume(t *testing.T) {
+	nonce, err := StoreSession("mcr", "https://mcr.example.com/cb", "state456")
+	if err != nil {
+		t.Fatalf("StoreSession: %v", err)
+	}
+	if nonce == "" {
+		t.Fatal("StoreSession returned empty nonce")
+	}
+
+	// GetSession should return it without consuming.
+	s := GetSession(nonce)
+	if s == nil {
+		t.Fatal("GetSession returned nil")
+	}
+	if s.ClientID != "mcr" {
+		t.Errorf("ClientID = %q", s.ClientID)
+	}
+
+	// Still available after GetSession.
+	s2, ok := ConsumeSession(nonce)
+	if !ok {
+		t.Fatal("ConsumeSession returned false")
+	}
+	if s2.State != "state456" {
+		t.Errorf("State = %q", s2.State)
+	}
+
+	// Consumed — should be gone.
+	if _, ok := ConsumeSession(nonce); ok {
+		t.Error("second ConsumeSession should fail")
+	}
+	if GetSession(nonce) != nil {
+		t.Error("GetSession should return nil after consume")
+	}
+}
+
+func TestConsumeSessionExpired(t *testing.T) {
+	nonce, err := StoreSession("mcr", "https://mcr.example.com/cb", "s")
+	if err != nil {
+		t.Fatalf("StoreSession: %v", err)
+	}
+
+	v, loaded := pendingSessions.Load(nonce)
+	if !loaded {
+		t.Fatal("session not found in pendingSessions")
+	}
+	sess, ok := v.(*Session)
+	if !ok {
+		t.Fatal("unexpected type in pendingSessions")
+	}
+	sess.ExpiresAt = time.Now().Add(-1 * time.Second)
+
+	if _, ok := ConsumeSession(nonce); ok {
+		t.Error("ConsumeSession should fail for expired session")
+	}
+}