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/mappers/repository.mapper.ts

@@ -0,0 +1,67 @@
+import { Repository, RepositoryDto, RepositoryEntity } from '$lib/server/models/repository.js';
+
+export class RepositoryMapper {
+	static fromEntity(entity: RepositoryEntity): Repository {
+		return new Repository({
+			id: entity.id,
+			title: entity.title,
+			description: entity.description,
+			source: entity.source,
+			sourceUrl: entity.source_url,
+			branch: entity.branch,
+			state: entity.state,
+			totalSnippets: entity.total_snippets ?? 0,
+			totalTokens: entity.total_tokens ?? 0,
+			trustScore: entity.trust_score ?? 0,
+			benchmarkScore: entity.benchmark_score ?? 0,
+			stars: entity.stars,
+			githubToken: entity.github_token,
+			lastIndexedAt:
+				entity.last_indexed_at != null ? new Date(entity.last_indexed_at * 1000) : null,
+			createdAt: new Date(entity.created_at * 1000),
+			updatedAt: new Date(entity.updated_at * 1000)
+		});
+	}
+
+	static toEntity(domain: Repository): RepositoryEntity {
+		return new RepositoryEntity({
+			id: domain.id,
+			title: domain.title,
+			description: domain.description,
+			source: domain.source,
+			source_url: domain.sourceUrl,
+			branch: domain.branch,
+			state: domain.state,
+			total_snippets: domain.totalSnippets,
+			total_tokens: domain.totalTokens,
+			trust_score: domain.trustScore,
+			benchmark_score: domain.benchmarkScore,
+			stars: domain.stars,
+			github_token: domain.githubToken,
+			last_indexed_at:
+				domain.lastIndexedAt != null ? Math.floor(domain.lastIndexedAt.getTime() / 1000) : null,
+			created_at: Math.floor(domain.createdAt.getTime() / 1000),
+			updated_at: Math.floor(domain.updatedAt.getTime() / 1000)
+		});
+	}
+
+	static toDto(domain: Repository): RepositoryDto {
+		return new RepositoryDto({
+			id: domain.id,
+			title: domain.title,
+			description: domain.description,
+			source: domain.source,
+			sourceUrl: domain.sourceUrl,
+			branch: domain.branch,
+			state: domain.state,
+			totalSnippets: domain.totalSnippets,
+			totalTokens: domain.totalTokens,
+			trustScore: domain.trustScore,
+			benchmarkScore: domain.benchmarkScore,
+			stars: domain.stars,
+			lastIndexedAt: domain.lastIndexedAt,
+			createdAt: domain.createdAt,
+			updatedAt: domain.updatedAt
+		});
+	}
+}