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/[id]/index/+server.ts

@@ -4,6 +4,7 @@
 import { json } from '@sveltejs/kit';
 import type { RequestHandler } from './$types';
 import { getClient } from '$lib/server/db/client';
+import { IndexingJobMapper } from '$lib/server/mappers/indexing-job.mapper.js';
 import { RepositoryService } from '$lib/server/services/repository.service';
 import { getQueue } from '$lib/server/pipeline/startup';
 import { handleServiceError, NotFoundError } from '$lib/server/utils/validation';
@@ -31,7 +32,7 @@ export const POST: RequestHandler = async ({ params, request }) => {
 			? queue.enqueue(id, versionId)
 			: service.createIndexingJob(id, versionId);
 
-		return json({ job }, { status: 202 });
+		return json({ job: IndexingJobMapper.toDto(job) }, { status: 202 });
 	} catch (err) {
 		return handleServiceError(err);
 	}