Migrate authentication to Better Auth

Replace the hand-rolled JWT auth with Better Auth 1.6.25 httpOnly cookie
sessions, validated against the database on every request so revocation,
bans and role changes take effect immediately.

Backend:
- betterAuth.ts wires the Drizzle adapter, magic links, Google sign-in and
  the admin plugin; auth-schema.ts maps Better Auth's models onto the
  existing `users` table so user IDs and their foreign keys survive intact.
- routes/auth.ts is gone; Better Auth serves the standard endpoints and
  authExt.ts carries the flows it doesn't cover.
- auth.ts shrinks to session resolution and helpers; sessions/revocation in
  dashboard.ts now read and delete `auth_sessions` rows directly.
- Schema adds the Better Auth core + admin columns (email_verified, image,
  banned, ban_reason, ban_expires), with migrations and tests.
- rateLimit.ts resolves client IPs spoof-resistantly: proxy headers are only
  honoured from loopback/RFC1918 peers plus TRUSTED_PROXIES.
- passwordPolicy.ts centralises password validation.
- Bump drizzle-orm, drizzle-kit and better-sqlite3 to versions compatible
  with Better Auth.

Frontend:
- auth-client.ts plus a reworked AuthContext and api/client.ts move to
  cookie-based sessions; no more bearer tokens in requests or middleware.

photo-api:
- Validate Better Auth session cookies against the shared auth_sessions
  table instead of verifying JWTs; JWT_SECRET is no longer needed for user
  auth, and PHOTO_VIEW_SECRET now signs gallery view tokens.

BETTER_AUTH_SECRET and BETTER_AUTH_URL are required in production; the
deprecated JWT_SECRET stays only as the photo-api view-token fallback.

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

frontend/src/app/admin/emails/page.tsx

@@ -107,11 +107,7 @@ export default function AdminEmailsPage() {
   
   const loadEvents = async () => {
     try {
-      const res = await fetch('/api/events', {
-        headers: {
-          'Authorization': `Bearer ${localStorage.getItem('spanglish-token')}`,
-        },
-      });
+      const res = await fetch('/api/events', { credentials: 'same-origin' });
       if (res.ok) {
         const data = await res.json();
         setEvents(data.events || []);
@@ -169,9 +165,7 @@ export default function AdminEmailsPage() {
     
     try {
       const res = await fetch(`/api/events/${composeForm.eventId}/attendees`, {
-        headers: {
-          'Authorization': `Bearer ${localStorage.getItem('spanglish-token')}`,
-        },
+        credentials: 'same-origin',
       });
       if (res.ok) {
         const data = await res.json();