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/config/config_test.go

@@ -244,6 +244,153 @@ func TestTrustedProxyValidation(t *testing.T) {
 	}
 }
 
+func TestSSOClientValidation(t *testing.T) {
+	tests := []struct {
+		name    string
+		extra   string
+		wantErr bool
+	}{
+		{
+			name: "valid single client",
+			extra: `
+[[sso.clients]]
+client_id    = "mcr"
+redirect_uri = "https://mcr.example.com/sso/callback"
+service_name = "mcr"
+tags         = ["env:restricted"]
+`,
+			wantErr: false,
+		},
+		{
+			name: "valid multiple clients",
+			extra: `
+[[sso.clients]]
+client_id    = "mcr"
+redirect_uri = "https://mcr.example.com/sso/callback"
+service_name = "mcr"
+
+[[sso.clients]]
+client_id    = "mcat"
+redirect_uri = "https://mcat.example.com/sso/callback"
+service_name = "mcat"
+`,
+			wantErr: false,
+		},
+		{
+			name: "missing client_id",
+			extra: `
+[[sso.clients]]
+redirect_uri = "https://mcr.example.com/sso/callback"
+service_name = "mcr"
+`,
+			wantErr: true,
+		},
+		{
+			name: "missing redirect_uri",
+			extra: `
+[[sso.clients]]
+client_id    = "mcr"
+service_name = "mcr"
+`,
+			wantErr: true,
+		},
+		{
+			name: "http redirect_uri rejected",
+			extra: `
+[[sso.clients]]
+client_id    = "mcr"
+redirect_uri = "http://mcr.example.com/sso/callback"
+service_name = "mcr"
+`,
+			wantErr: true,
+		},
+		{
+			name: "missing service_name",
+			extra: `
+[[sso.clients]]
+client_id    = "mcr"
+redirect_uri = "https://mcr.example.com/sso/callback"
+`,
+			wantErr: true,
+		},
+		{
+			name: "duplicate client_id",
+			extra: `
+[[sso.clients]]
+client_id    = "mcr"
+redirect_uri = "https://mcr.example.com/sso/callback"
+service_name = "mcr"
+
+[[sso.clients]]
+client_id    = "mcr"
+redirect_uri = "https://other.example.com/sso/callback"
+service_name = "mcr2"
+`,
+			wantErr: true,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			path := writeTempConfig(t, validConfig()+tc.extra)
+			_, err := Load(path)
+			if tc.wantErr && err == nil {
+				t.Error("expected validation error, got nil")
+			}
+			if !tc.wantErr && err != nil {
+				t.Errorf("unexpected error: %v", err)
+			}
+		})
+	}
+}
+
+func TestSSOClientLookup(t *testing.T) {
+	path := writeTempConfig(t, validConfig()+`
+[[sso.clients]]
+client_id    = "mcr"
+redirect_uri = "https://mcr.example.com/sso/callback"
+service_name = "mcr"
+tags         = ["env:restricted"]
+`)
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+
+	cl := cfg.SSOClient("mcr")
+	if cl == nil {
+		t.Fatal("SSOClient(mcr) returned nil")
+	}
+	if cl.RedirectURI != "https://mcr.example.com/sso/callback" {
+		t.Errorf("RedirectURI = %q", cl.RedirectURI)
+	}
+	if cl.ServiceName != "mcr" {
+		t.Errorf("ServiceName = %q", cl.ServiceName)
+	}
+	if len(cl.Tags) != 1 || cl.Tags[0] != "env:restricted" {
+		t.Errorf("Tags = %v", cl.Tags)
+	}
+
+	if cfg.SSOClient("nonexistent") != nil {
+		t.Error("SSOClient(nonexistent) should return nil")
+	}
+
+	if !cfg.SSOEnabled() {
+		t.Error("SSOEnabled() should return true")
+	}
+}
+
+func TestSSODisabledByDefault(t *testing.T) {
+	path := writeTempConfig(t, validConfig())
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if cfg.SSOEnabled() {
+		t.Error("SSOEnabled() should return false with no clients")
+	}
+}
+
 func TestDurationParsing(t *testing.T) {
 	var d duration
 	if err := d.UnmarshalText([]byte("1h30m")); err != nil {