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]/+server.ts

@@ -6,6 +6,7 @@
 import { json } from '@sveltejs/kit';
 import type { RequestHandler } from './$types';
 import { getClient } from '$lib/server/db/client';
+import { RepositoryMapper } from '$lib/server/mappers/repository.mapper.js';
 import { RepositoryService } from '$lib/server/services/repository.service';
 import { handleServiceError } from '$lib/server/utils/validation';
 
@@ -22,8 +23,7 @@ export const GET: RequestHandler = ({ params }) => {
 			return json({ error: 'Repository not found', code: 'NOT_FOUND' }, { status: 404 });
 		}
 		const versions = service.getVersions(id);
-		const { githubToken: _token, ...safeRepo } = repo;
-		return json({ ...safeRepo, versions });
+		return json({ ...RepositoryMapper.toDto(repo), versions });
 	} catch (err) {
 		return handleServiceError(err);
 	}
@@ -40,8 +40,7 @@ export const PATCH: RequestHandler = async ({ params, request }) => {
 			branch: body.branch,
 			githubToken: body.githubToken
 		});
-		const { githubToken: _token, ...safeUpdated } = updated;
-		return json(safeUpdated);
+		return json(RepositoryMapper.toDto(updated));
 	} catch (err) {
 		return handleServiceError(err);
 	}