fix(admin-users): normalise date fields to ISO across DB drivers (#485)

Admin > Users page crashed with "TypeError: e.split is not a function"
on native installs (SQLite default). Reported by @blazmaric in #485
with a clean diagnosis: SQLite returns lastLogin / createdAt /
updatedAt as integer milliseconds since epoch, while Postgres
returns ISO strings via the standard JSON serialiser. The page used
parseISO() on the raw value and parseISO trips on numbers.

Fix at both layers — defence in depth:

- backend/src/routes/adminUsers.js: new toIso() helper applied in
  transformUser + transformInvitation. Coerces Date / number /
  numeric-string / null to a single ISO 8601 string contract before
  the response leaves the API. Protects every consumer (frontend
  AND external API tokens / n8n) regardless of which DB driver is
  underneath.
- frontend/src/services/userManagement.service.ts: same helper as
  defence-in-depth for stale backends mid-deploy and any cached
  pre-fix response shape. Also surfaced an existing
  transformInvitation gap — invitations endpoints were returning
  raw response.data.invitations without going through the
  transformer.

10 unit tests pin the toIso contract: all known driver shapes
(Date, number, numeric-string, ISO-string, null/undefined/empty)
plus the full transformer paths for transformUser and
transformInvitation.

Out of scope: same epoch-ms surface may exist on other admin pages
that were never tested against SQLite (events list, customers,
webhooks, api tokens, activity log). Worth a follow-up audit pass
to apply toIso() in every snake_case→camelCase transformer the
admin routes use, but the immediate Users-page crash is the only
reported one and shipping that fix unblocks @blazmaric.

backend/src/routes/adminUsers.js

@@ -11,6 +11,37 @@ const { handleAsync, validateRequest, successResponse } = require('../utils/rout
 const userManagementService = require('../services/userManagementService');
 const router = express.Router();
 
+/**
+ * Coerce any of the shapes a TIMESTAMP column produces across our
+ * supported drivers into a single ISO 8601 string the frontend (and
+ * any external API consumer) can safely pass to date-fns / new Date.
+ *
+ * Postgres → Date object (becomes ISO via JSON.stringify anyway, but
+ *   pinning the format defends against driver-side surprises).
+ * SQLite → integer milliseconds since epoch (the surface that crashed
+ *   the admin Users page in #485 — `parseISO(123456789)` blows up
+ *   with "e.split is not a function"). Native installs default to
+ *   SQLite, so this path matters every release.
+ * Already a string → assume it's a parseable ISO/RFC3339 (Postgres
+ *   driver may stringify under JSON serialization mid-pipeline).
+ *
+ * Returns null/undefined unchanged so an unset last_login surfaces as
+ * "Never" in the UI rather than 1970-01-01T00:00:00Z.
+ */
+function toIso(value) {
+  if (value === null || value === undefined || value === '') return value;
+  if (value instanceof Date) return value.toISOString();
+  if (typeof value === 'number') return new Date(value).toISOString();
+  if (typeof value === 'string') {
+    // Numeric-as-string ("1778752458666") happens when the SQLite
+    // driver stringifies large integers — re-coerce so the frontend
+    // doesn't try to parseISO('1778752458666').
+    if (/^\d{10,}$/.test(value)) return new Date(Number(value)).toISOString();
+    return value;
+  }
+  return value;
+}
+
 /**
  * Transform user object from snake_case (DB) to camelCase (API)
  */
@@ -20,10 +51,10 @@ function transformUser(user) {
     username: user.username,
     email: user.email,
     isActive: user.is_active,
-    lastLogin: user.last_login,
+    lastLogin: toIso(user.last_login),
     lastLoginIp: user.last_login_ip,
-    createdAt: user.created_at,
-    updatedAt: user.updated_at,
+    createdAt: toIso(user.created_at),
+    updatedAt: toIso(user.updated_at),
     roleId: user.role_id,
     roleName: user.role_name,
     roleDisplayName: user.role_display_name,
@@ -52,8 +83,8 @@ function transformInvitation(invitation) {
   return {
     id: invitation.id,
     email: invitation.email,
-    expiresAt: invitation.expires_at,
-    createdAt: invitation.created_at,
+    expiresAt: toIso(invitation.expires_at),
+    createdAt: toIso(invitation.created_at),
     roleName: invitation.role_name,
     invitedBy: invitation.invited_by
   };
@@ -212,4 +243,7 @@ router.post('/:id/reset-password', [
   successResponse(res, { message: 'Password reset email sent', ...result });
 }));
 
+// Test surface: expose the date normaliser so the unit test can pin
+// the contract without spinning up the full router.
 module.exports = router;
+module.exports.__test = { toIso, transformUser, transformInvitation };