feat(auth): email login soft-disabled until SMTP/Resend is wired

Closes the dependency on an unbuilt email sender. New EMAIL_AUTH_ENABLED
env flag (default false). When off:

- POST /v1/auth/magic-link  → 503 email_auth_disabled
- POST /v1/auth/verify       → 503 email_auth_disabled
- GET  /v1/auth/providers    → { email: false, sms, google, github }
- Login page: hides the email/phone tab toggle (only one method),
  hides the email form entirely, defaults to SMS/phone tab

Flipping EMAIL_AUTH_ENABLED=true re-enables the magic-link routes and
re-shows the email form section. Schema (magic_links table) unchanged
so this is a 1-env-flip re-enable, not a re-implementation.

SECURITY: closes audit finding Za-001 (account-takeover via
cross-provider email lookup). Without a magic-link flow, an attacker
who controls a target's inbox can no longer claim an existing
OAuth-created account. The remaining provider-mixing surface (Google
↔ GitHub at same email) requires controlling the OAuth provider
account itself, which is each provider's own security boundary.

Active login methods now: Google OAuth · GitHub OAuth · SMS code
(Twilio) · admin password (seeded, single user).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/api/src/routes/auth.ts

@@ -76,6 +76,16 @@ function smsIpRateOk(ip: string, max = 5, windowMs = 10 * 60 * 1000): boolean {
 
 export async function authRoutes(app: FastifyInstance): Promise<void> {
   app.post('/v1/auth/magic-link', async (req, reply) => {
+    // Email auth is off by default — no SMTP wired yet. Closes the
+    // account-takeover-via-magic-link path (Za-001) until an email sender
+    // is configured AND a primaryProvider column lets us bind users to a
+    // single login method.
+    if (!config.EMAIL_AUTH_ENABLED) {
+      return reply.code(503).send({
+        error: 'email_auth_disabled',
+        detail: 'Email login is currently unavailable. Use Google, GitHub, or SMS.',
+      });
+    }
     const Body = z.object({ email: z.string().email() });
     const parsed = Body.safeParse(req.body);
     if (!parsed.success) return reply.code(400).send({ error: 'invalid_email' });
@@ -124,6 +134,9 @@ export async function authRoutes(app: FastifyInstance): Promise<void> {
   });
 
   app.post('/v1/auth/verify', async (req, reply) => {
+    if (!config.EMAIL_AUTH_ENABLED) {
+      return reply.code(503).send({ error: 'email_auth_disabled' });
+    }
     const Body = z.object({ token: z.string().min(10) });
     const parsed = Body.safeParse(req.body);
     if (!parsed.success) return reply.code(400).send({ error: 'invalid_token' });
@@ -229,13 +242,16 @@ export async function authRoutes(app: FastifyInstance): Promise<void> {
     return reply.send({ ok: true });
   });
 
-  // Which third-party login providers are configured. Lets the UI hide the
-  // Google button when no credentials are set, instead of showing a dead button.
+  // Which login providers are configured. Lets the UI hide buttons + forms
+  // when their backing infra isn't wired. `email` defaults to false because
+  // we haven't bought an SMTP provider yet — flipping EMAIL_AUTH_ENABLED to
+  // true re-enables the magic-link form section.
   app.get('/v1/auth/providers', async (_req, reply) => {
     return reply.send({
       google: googleConfigured(),
       github: githubConfigured(),
       sms: smsConfigured(),
+      email: config.EMAIL_AUTH_ENABLED,
     });
   });