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/lib/api/client.ts

@@ -1,30 +1,25 @@
 export const API_BASE = process.env.NEXT_PUBLIC_API_URL || '';
 
+// Auth rides on the Better Auth httpOnly session cookie. With API_BASE unset
+// (same-origin via Next rewrites) 'same-origin' sends it; a cross-origin
+// API_BASE needs 'include' plus CORS credentials on the backend.
+const CREDENTIALS: RequestCredentials = API_BASE ? 'include' : 'same-origin';
+
 export interface ApiError {
   error: string;
 }
 
-/** Read the stored auth token (browser only). */
-export function getToken(): string | null {
-  return typeof window !== 'undefined' ? localStorage.getItem('spanglish-token') : null;
-}
-
 export async function fetchApi<T>(
   endpoint: string,
   options: RequestInit = {}
 ): Promise<T> {
-  const token = getToken();
-
   const headers: HeadersInit = {
     'Content-Type': 'application/json',
     ...options.headers,
   };
 
-  if (token) {
-    (headers as Record<string, string>)['Authorization'] = `Bearer ${token}`;
-  }
-
   const res = await fetch(`${API_BASE}${endpoint}`, {
+    credentials: CREDENTIALS,
     ...options,
     headers,
   });
@@ -53,11 +48,7 @@ export async function fetchBlob(
   endpoint: string,
   fallbackFilename: string
 ): Promise<{ blob: Blob; filename: string }> {
-  const token = getToken();
-  const headers: Record<string, string> = {};
-  if (token) headers['Authorization'] = `Bearer ${token}`;
-
-  const res = await fetch(`${API_BASE}${endpoint}`, { headers });
+  const res = await fetch(`${API_BASE}${endpoint}`, { credentials: CREDENTIALS });
   if (!res.ok) {
     const errorData = await res.json().catch(() => ({ error: 'Export failed' }));
     throw new Error(errorData.error || 'Export failed');