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/lib/server/pipeline/indexing.pipeline.ts

@@ -15,10 +15,13 @@
 
 import { createHash } from 'node:crypto';
 import type Database from 'better-sqlite3';
-import type { Document, IndexingJob, NewDocument, NewSnippet, Repository } from '$lib/types';
+import type { Document, NewDocument, NewSnippet } from '$lib/types';
 import type { crawl as GithubCrawlFn } from '$lib/server/crawler/github.crawler.js';
 import type { LocalCrawler } from '$lib/server/crawler/local.crawler.js';
 import type { EmbeddingService } from '$lib/server/embeddings/embedding.service.js';
+import { RepositoryMapper } from '$lib/server/mappers/repository.mapper.js';
+import { IndexingJob } from '$lib/server/models/indexing-job.js';
+import { Repository, RepositoryEntity } from '$lib/server/models/repository.js';
 import { parseFile } from '$lib/server/parser/index.js';
 import { computeTrustScore } from '$lib/server/search/trust-score.js';
 import { computeDiff } from './diff.js';
@@ -399,11 +402,10 @@ export class IndexingPipeline {
 	}
 
 	private getRepository(id: string): Repository | null {
-		return (
-			(this.db
-				.prepare<[string], Repository>(`SELECT * FROM repositories WHERE id = ?`)
-				.get(id) as Repository | undefined) ?? null
-		);
+		const raw = this.db
+			.prepare<[string], RepositoryEntity>(`SELECT * FROM repositories WHERE id = ?`)
+			.get(id);
+		return raw ? RepositoryMapper.fromEntity(new RepositoryEntity(raw)) : null;
 	}
 
 	private updateJob(id: string, fields: Record<string, unknown>): void {