Add session revocation & consolidate rate limiting

Add sessions_invalidated_at to users (migration + Prisma schema) to support immediate session revocation. Set sessionsInvalidatedAt when an admin revokes a user's login token and enforce revocation checks in auth middleware and the refresh endpoint (compare token iat against sessionsInvalidatedAt). Add optional iat fields to JWT payload types. Scrub token from browser history after token-login. Consolidate rate-limiting logic into src/lib/utils/rateLimit.ts (rename/merge previous auth/apiToken rate limiter implementations), remove the old apiTokenRateLimit.ts, and update imports and tests to use the new module.

prisma/schema.prisma

@@ -60,6 +60,9 @@ model User {
   // Login token (admin-generated, for direct URL login)
   loginTokenHash String? @map("login_token_hash") // SHA-256 hash of the login token (never store plaintext)
 
+  // Session invalidation (set when login token is revoked to force-logout active sessions)
+  sessionsInvalidatedAt DateTime? @map("sessions_invalidated_at")
+
   // Soft delete support
   deletedAt DateTime? @map("deleted_at")
   deletedBy String?   @map("deleted_by") // Admin user ID who deleted this user