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.go

@@ -0,0 +1,93 @@
+// Package sso implements the authorization code store for the SSO redirect flow.
+//
+// MCIAS acts as the SSO provider: downstream services (MCR, MCAT, Metacrypt)
+// redirect users to MCIAS for login, and MCIAS issues a short-lived, single-use
+// authorization code that the service exchanges for a JWT token.
+//
+// Security design:
+//   - Authorization codes are 32 random bytes (256 bits), hex-encoded.
+//   - Codes are single-use: consumed via sync.Map LoadAndDelete on first exchange.
+//   - Codes expire after 60 seconds to limit the window for interception.
+//   - A background goroutine evicts expired codes every 5 minutes.
+//   - The code is bound to the client_id and redirect_uri presented at authorize
+//     time; the token exchange endpoint must verify both match.
+package sso
+
+import (
+	"fmt"
+	"sync"
+	"time"
+
+	"git.wntrmute.dev/mc/mcias/internal/crypto"
+)
+
+const (
+	codeTTL       = 60 * time.Second
+	codeBytes     = 32 // 256 bits of entropy
+	cleanupPeriod = 5 * time.Minute
+)
+
+// AuthCode is a pending authorization code awaiting exchange for a JWT.
+type AuthCode struct { //nolint:govet // fieldalignment: field order matches logical grouping
+	ClientID    string
+	RedirectURI string
+	State       string
+	AccountID   int64
+	ExpiresAt   time.Time
+}
+
+// pendingCodes stores issued authorization codes awaiting exchange.
+var pendingCodes sync.Map //nolint:gochecknoglobals
+
+func init() {
+	go cleanupCodes()
+}
+
+func cleanupCodes() {
+	ticker := time.NewTicker(cleanupPeriod)
+	defer ticker.Stop()
+	for range ticker.C {
+		now := time.Now()
+		pendingCodes.Range(func(key, value any) bool {
+			ac, ok := value.(*AuthCode)
+			if !ok || now.After(ac.ExpiresAt) {
+				pendingCodes.Delete(key)
+			}
+			return true
+		})
+	}
+}
+
+// Store creates and stores a new authorization code bound to the given
+// client_id, redirect_uri, state, and account. Returns the hex-encoded code.
+func Store(clientID, redirectURI, state string, accountID int64) (string, error) {
+	raw, err := crypto.RandomBytes(codeBytes)
+	if err != nil {
+		return "", fmt.Errorf("sso: generate authorization code: %w", err)
+	}
+	code := fmt.Sprintf("%x", raw)
+	pendingCodes.Store(code, &AuthCode{
+		ClientID:    clientID,
+		RedirectURI: redirectURI,
+		State:       state,
+		AccountID:   accountID,
+		ExpiresAt:   time.Now().Add(codeTTL),
+	})
+	return code, nil
+}
+
+// Consume retrieves and deletes an authorization code. Returns the AuthCode
+// and true if the code was valid and not expired, or (nil, false) otherwise.
+//
+// Security: LoadAndDelete ensures single-use; the code cannot be replayed.
+func Consume(code string) (*AuthCode, bool) {
+	v, ok := pendingCodes.LoadAndDelete(code)
+	if !ok {
+		return nil, false
+	}
+	ac, ok2 := v.(*AuthCode)
+	if !ok2 || time.Now().After(ac.ExpiresAt) {
+		return nil, false
+	}
+	return ac, true
+}