feat(api): add per-type file size limits and 413 responses for media uploads

- FileValidationPipe now supports maxSizeByMimeType for per-MIME-type size limits
- Images: max 10MB, Video (MP4): max 100MB
- Oversized files return 413 Payload Too Large instead of 400 Bad Request
- MIME type validation runs before size check for clearer error messages
- Multer module limit raised to 100MB (per-type enforcement in pipe)
- Added 413 ApiResponse to Swagger docs on upload endpoint
- Added comprehensive unit tests for FileValidationPipe (16 test cases)

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/src/modules/listings/listings.module.ts

@@ -37,7 +37,7 @@ const QueryHandlers = [
   imports: [
     CqrsModule,
     MulterModule.register({
-      limits: { fileSize: 10 * 1024 * 1024 }, // 10 MB
+      limits: { fileSize: 100 * 1024 * 1024 }, // 100 MB — per-type limits enforced by FileValidationPipe
     }),
   ],
   controllers: [ListingsController],