refactor(backend): hoist parseIntParam helper to utils (#798)

Adds backend/src/utils/parseIntParam.ts with a shared parseIntParam
helper that writes a 400 'Invalid <label>' response and returns null on
non-numeric route params. Consolidates the parseInt + isNaN + 400 shape
that was inlined or duplicated across multiple routers.

Updated:
- routes/fleet.ts: replaced the local parseIdParam wrapper.
- routes/autoHeal.ts: replaced the local parsePolicyId wrapper.
- routes/notifications.ts: replaced parseRouteId wrapper plus an inline
  notification-id site.
- routes/apiTokens.ts, routes/labels.ts, routes/registries.ts,
  routes/scheduledTasks.ts, routes/users.ts: replaced inline copies.

Out of scope (route handlers without an existing isNaN check, kept
intentionally untouched to avoid introducing new 400 responses): alerts,
nodes, webhooks, and several user-routes handlers that rely on a
downstream 404 instead.

Closes #748

backend/src/utils/parseIntParam.ts

@@ -0,0 +1,24 @@
+import type { Request, Response } from 'express';
+
+/**
+ * Parse a numeric route param. Writes a 400 response and returns null when
+ * the value isn't a valid integer; callers early-return on null.
+ *
+ * @param paramName the key in req.params (e.g. 'id', 'nodeId')
+ * @param label optional human-readable label used in the error body.
+ *   Falls back to paramName when omitted (e.g. 'Invalid id').
+ */
+export function parseIntParam(
+  req: Request,
+  res: Response,
+  paramName: string,
+  label?: string,
+): number | null {
+  const raw = req.params[paramName] as string | undefined;
+  const parsed = parseInt(raw ?? '', 10);
+  if (isNaN(parsed)) {
+    res.status(400).json({ error: `Invalid ${label ?? paramName}` });
+    return null;
+  }
+  return parsed;
+}