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/context/+server.ts

@@ -13,6 +13,7 @@
 
 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 { HybridSearchService } from '$lib/server/search/hybrid.search.service';
 import { parseLibraryId } from '$lib/server/api/library-id';
@@ -186,9 +187,9 @@ export const GET: RequestHandler = async ({ url }) => {
 
 		// Default: JSON
 		const body = formatContextJson(selectedResults, rules);
-		return new Response(JSON.stringify(body), {
+		return dtoJsonResponse(body, {
 			status: 200,
-			headers: { 'Content-Type': 'application/json', ...CORS_HEADERS }
+			headers: CORS_HEADERS
 		});
 	} catch (err) {
 		const message = err instanceof Error ? err.message : 'Internal server error';