fix(api): serialize BigInt in all API success responses

successResponse() used NextResponse.json(), which calls JSON.stringify and
throws on BigInt. Prisma returns BigInt for VideoVersion.sizeBytes and
VideoAsset.sizeBytes, so any route returning one of those rows returned 500
after its database write had already committed.

#27 fixed two such endpoints by narrowing their selects, and two create
routes were already wrapped in toJsonSafe(). This closes the bug class at
the helper instead: successResponse() now serializes with a shared
bigIntReplacer, which covers every route in app/api (none construct a
NextResponse.json response directly).

The two toJsonSafe() call sites are now redundant and were removed. BigInt
values render as strings, matching what toJsonSafe already produced.

lib/api-response.ts

@@ -1,4 +1,5 @@
 import { NextResponse } from 'next/server';
+import { bigIntReplacer } from '@/lib/json-serialize';
 
 /**
  * Standardized API error response format
@@ -113,6 +114,11 @@ export function errorResponse(
  * @param status - HTTP status code (default: 200)
  * @param meta - Pagination or other metadata (optional)
  *
+ * Serialized with bigIntReplacer rather than NextResponse.json(), because
+ * JSON.stringify throws on BigInt and Prisma returns BigInt for sizeBytes.
+ * Any payload carrying a VideoVersion or VideoAsset row would otherwise 500
+ * after its write had already committed. BigInt values render as strings.
+ *
  * @example
  * ```ts
  * return successResponse({ projects: [] });
@@ -127,7 +133,10 @@ export function successResponse<T>(
   const body: ApiSuccessResponse<T> = { data };
   if (meta) body.meta = meta;
 
-  return NextResponse.json(body, { status });
+  return new NextResponse(JSON.stringify(body, bigIntReplacer), {
+    status,
+    headers: { 'content-type': 'application/json' },
+  }) as NextResponse<ApiSuccessResponse<T>>;
 }
 
 export function withCacheControl(response: Response, value: string): Response {

lib/json-serialize.ts

@@ -1,8 +1,18 @@
+/**
+ * JSON.stringify replacer that renders Prisma BigInt columns (sizeBytes) as
+ * strings. Without it, JSON.stringify throws on any payload carrying one.
+ */
+export function bigIntReplacer(_key: string, value: unknown): unknown {
+  return typeof value === 'bigint' ? value.toString() : value;
+}
+
 /**
  * Converts values for JSON responses (e.g. Prisma BigInt fields).
+ *
+ * API routes do not need this — successResponse() serializes BigInt already.
+ * Use it for payloads that bypass that helper, such as data handed from a
+ * server component to a client component.
  */
 export function toJsonSafe<T>(value: T): T {
-  return JSON.parse(
-    JSON.stringify(value, (_key, val) => (typeof val === 'bigint' ? val.toString() : val))
-  ) as T;
+  return JSON.parse(JSON.stringify(value, bigIntReplacer)) as T;
 }