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/indexing-job.mapper.ts

@@ -0,0 +1,36 @@
+import { IndexingJob, IndexingJobDto, IndexingJobEntity } from '$lib/server/models/indexing-job.js';
+
+export class IndexingJobMapper {
+	static fromEntity(entity: IndexingJobEntity): IndexingJob {
+		return new IndexingJob({
+			id: entity.id,
+			repositoryId: entity.repository_id,
+			versionId: entity.version_id,
+			status: entity.status,
+			progress: entity.progress,
+			totalFiles: entity.total_files,
+			processedFiles: entity.processed_files,
+			error: entity.error,
+			startedAt: entity.started_at != null ? new Date(entity.started_at * 1000) : null,
+			completedAt:
+				entity.completed_at != null ? new Date(entity.completed_at * 1000) : null,
+			createdAt: new Date(entity.created_at * 1000)
+		});
+	}
+
+	static toDto(domain: IndexingJob): IndexingJobDto {
+		return new IndexingJobDto({
+			id: domain.id,
+			repositoryId: domain.repositoryId,
+			versionId: domain.versionId,
+			status: domain.status,
+			progress: domain.progress,
+			totalFiles: domain.totalFiles,
+			processedFiles: domain.processedFiles,
+			error: domain.error,
+			startedAt: domain.startedAt,
+			completedAt: domain.completedAt,
+			createdAt: domain.createdAt
+		});
+	}
+}