feat(server): api-helpers fuer parsePositiveIntParam + validateBody

- src/lib/server/api-helpers.ts mit parsePositiveIntParam(),
  validateBody<T>() und ErrorResponse type
- 13 unit tests fuer die beiden helper (HttpError-Shape verifiziert)
- Konsolidiert spaeter 9x parseId und 11x safeParse-Bloecke aus den
  +server.ts handlern

Findings aus REVIEW-2026-04-18.md (Refactor A) und redundancy.md

src/lib/server/api-helpers.ts

@@ -0,0 +1,39 @@
+import { error } from '@sveltejs/kit';
+import type { ZodSchema } from 'zod';
+
+// Shared error body shape for SvelteKit `error()` calls. `issues` is set
+// when validateBody fails so the client can show a precise validation
+// hint; everywhere else only `message` is used.
+export type ErrorResponse = {
+  message: string;
+  issues?: unknown;
+};
+
+/**
+ * Parse a route param (or query param) as a positive integer (>=1).
+ * Throws SvelteKit `error(400)` with `Missing <field>` when null/undefined,
+ * or `Invalid <field>` when the value is not an integer >= 1.
+ */
+export function parsePositiveIntParam(
+  raw: string | undefined | null,
+  field: string
+): number {
+  if (raw == null) error(400, { message: `Missing ${field}` });
+  const n = Number(raw);
+  if (!Number.isInteger(n) || n <= 0) error(400, { message: `Invalid ${field}` });
+  return n;
+}
+
+/**
+ * Validate an unknown body against a Zod schema. Throws SvelteKit
+ * `error(400, { message: 'Invalid body', issues })` on mismatch and returns
+ * the typed parse result on success. Accepts `null` (the typical result of
+ * `await request.json().catch(() => null)`).
+ */
+export function validateBody<T>(body: unknown, schema: ZodSchema<T>): T {
+  const parsed = schema.safeParse(body);
+  if (!parsed.success) {
+    error(400, { message: 'Invalid body', issues: parsed.error.issues });
+  }
+  return parsed.data;
+}