refactor: introduce domain model classes and mapper layer

Replace ad-hoc inline row casting (snake_case → camelCase) spread across
services, routes, and the indexing pipeline with explicit model classes
(Repository, IndexingJob, RepositoryVersion, Snippet, SearchResult) and
dedicated mapper classes that own the DB → domain conversion.

- Add src/lib/server/models/ with typed model classes for all domain entities
- Add src/lib/server/mappers/ with mapper classes per entity
- Remove duplicated RawRow interfaces and inline map functions from
  job-queue, repository.service, indexing.pipeline, and all API routes
- Add dtoJsonResponse helper to standardise JSON responses via SvelteKit json()
- Add api-contract.integration.test.ts as a regression baseline

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/routes/api/v1/libs/search/+server.ts

@@ -14,6 +14,7 @@
 import { json } from '@sveltejs/kit';
 import type { RequestHandler } from './$types';
 import { getClient } from '$lib/server/db/client';
+import { dtoJsonResponse } from '$lib/server/api/dto-response';
 import { SearchService } from '$lib/server/search/search.service';
 import { formatLibrarySearchJson } from '$lib/server/api/formatters';
 import { CORS_HEADERS } from '$lib/server/api/formatters';
@@ -44,7 +45,7 @@ export const GET: RequestHandler = ({ url }) => {
 		const results = service.searchRepositories({ libraryName, query, limit });
 		const body = formatLibrarySearchJson(results);
 
-		return json(body, {
+		return dtoJsonResponse(body, {
 			headers: CORS_HEADERS
 		});
 	} catch (err) {