Add admin identity module: better-auth, per-app permissions, invitations

Introduces src/models/admin/, a dedicated identity and permissions module on
its own nachklang_admin database, and the shared authenticator that feedback
and tickets will move onto in the cutover step. Nothing swaps over yet:
feedback.auth.ts and tickets.auth.ts still authenticate against the legacy
calendar sessions, so production behaviour is unchanged.

- better-auth 1.7 mounted at /admin/auth/*, sessions as httpOnly cookies
  scoped to .nachklang.art so one sign-in covers every *.nachklang.art app.
- Accounts are invite-only: public sign-up is disabled, and the invitations
  plugin is the only code that creates users. Tokens are stored as SHA-256
  hashes and travel in the request body, never in a URL.
- Per-app permissions in user_app_permissions; requireAppAccess(app) queries
  the database on every request (no cookie cache) so disabling a user or
  revoking a session takes effect immediately.
- ADMIN_BOOTSTRAP_EMAIL guarantees a way in on an empty database, idempotently
  and without crashing the API if the database is unreachable at boot.
- Guards prevent an admin from removing their own admin permission, disabling
  themselves, or stripping the last active admin.

The admin pool uses the callback-style mysql2, not mysql2/promise: Kysely's
MysqlDialect drives the pool with callbacks, and the promise wrapper ignores
them, so every query hangs silently. Only the integration tests caught this.

Schema in sql/admin/001_init.sql, derived from getAuthTables() on the
installed better-auth rather than the published CLI, which lags the library
and omits account.issuer.

app.ts is split into src/app.factory.ts so the integration tests drive the
real middleware order rather than a copy of it.

Tests: 131 unit, plus 41 integration tests against a throwaway MariaDB
started by test/integration/setup.ts (docker or podman).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/models/admin/admin.auth.ts

@@ -0,0 +1,141 @@
+import {betterAuth} from 'better-auth';
+import {APIError} from 'better-auth/api';
+import {passkey} from '@better-auth/passkey';
+import {NachklangAdminDB} from './Admin.db.js';
+import {invitationsPlugin} from './invitations/invitations.plugin.js';
+import {sendPasswordResetMail} from './admin.mail.js';
+import * as UsersService from './users/users.admin.service.js';
+import logger from '../../middleware/logger.js';
+import {
+	ADMIN_ALLOWED_ORIGINS,
+	API_BASE_URL,
+	BETTER_AUTH_SECRET,
+	PASSKEY_RP_ID,
+	isProd
+} from './admin.config.js';
+
+/**
+ * The single better-auth instance for all *.nachklang.art apps. Mounted in
+ * app.ts at /admin/auth/* with better-auth's own node handler, ahead of
+ * express.json() (it needs the raw body stream).
+ *
+ * The session cookie is what every app trusts. Everything else in this module -
+ * permissions, invitations, the admin UI - hangs off it.
+ */
+
+const DAY = 60 * 60 * 24;
+
+// Dev runs the apps on plain localhost ports; cookies ignore the port, so
+// single-sign-on across them works without fake subdomains or mkcert.
+const localhostOrigins = [
+	'http://localhost:3000',
+	'http://localhost:3001',
+	'http://localhost:3002',
+	'http://localhost:3003'
+];
+
+const trustedOrigins = isProd
+	? ADMIN_ALLOWED_ORIGINS
+	: Array.from(new Set([...ADMIN_ALLOWED_ORIGINS, ...localhostOrigins]));
+
+export const auth = betterAuth({
+	appName: 'Nachklang',
+	database: {
+		dialect: NachklangAdminDB.dialect,
+		type: 'mysql'
+	},
+	basePath: '/admin/auth',
+	// Mandatory once crossSubDomainCookies is on: better-auth derives the
+	// cookie domain and its own absolute URLs from this.
+	baseURL: API_BASE_URL,
+	secret: BETTER_AUTH_SECRET,
+	trustedOrigins,
+
+	emailAndPassword: {
+		enabled: true,
+		// There is no public sign-up: accounts exist only through an
+		// invitation (see invitations.plugin.ts). This also makes
+		// auth.api.signUpEmail throw, which is intended.
+		disableSignUp: true,
+		sendResetPassword: async ({user, url}) => {
+			await sendPasswordResetMail(user.email, user.name, url);
+		}
+	},
+
+	user: {
+		additionalFields: {
+			// Not `returned`, and not settable through the API: disabling is an
+			// admin action on /admin/users/:id/disable, never something a
+			// session owner can flip on themselves.
+			disabled: {
+				type: 'boolean',
+				defaultValue: false,
+				input: false,
+				returned: false
+			}
+		}
+	},
+
+	session: {
+		expiresIn: 30 * DAY,
+		updateAge: DAY
+		// Deliberately no cookieCache: requireAppAccess hits the database on
+		// every request anyway, and a cached session would keep a disabled
+		// user or a revoked session alive for the cache's lifetime.
+	},
+
+	advanced: {
+		// Fixes the cookie name across releases so the frontends' middleware can
+		// check for it: "nachklang.session_token", or
+		// "__Secure-nachklang.session_token" over https.
+		cookiePrefix: 'nachklang',
+		crossSubDomainCookies: isProd
+			? {enabled: true, domain: '.nachklang.art'}
+			: {enabled: false},
+		ipAddress: {
+			// better-auth reads the request itself and does not know about
+			// Express's `trust proxy`, so the header has to be named here.
+			// Verify against what Plesk's nginx actually sets before relying on
+			// the rate limiter (see the plan's pre-deploy checklist).
+			ipAddressHeaders: ['x-real-ip', 'x-forwarded-for']
+		}
+	},
+
+	rateLimit: {
+		enabled: true,
+		// Passenger may run more than one instance; an in-memory limiter would
+		// then give each of them its own budget.
+		storage: 'database'
+	},
+
+	plugins: [
+		passkey({
+			rpID: PASSKEY_RP_ID,
+			rpName: 'Nachklang',
+			origin: ADMIN_ALLOWED_ORIGINS
+		}),
+		invitationsPlugin()
+	],
+
+	databaseHooks: {
+		session: {
+			create: {
+				before: async session => {
+					const access = await UsersService.loadAccess(session.userId);
+					if (access?.disabled) {
+						logger.warn('Admin: sign-in attempt by a disabled account', {userId: session.userId});
+						// Throwing rather than returning false: `false` aborts
+						// the session write silently and the caller sees a
+						// confusing success-shaped response with no cookie.
+						throw new APIError('FORBIDDEN', {
+							code: 'ACCOUNT_DISABLED',
+							message: 'Dieses Konto ist deaktiviert.'
+						});
+					}
+				}
+			}
+		}
+	}
+});
+
+export type AdminAuth = typeof auth;