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-version.mapper.ts

@@ -0,0 +1,33 @@
+import {
+	RepositoryVersion,
+	RepositoryVersionDto,
+	RepositoryVersionEntity
+} from '$lib/server/models/repository-version.js';
+
+export class RepositoryVersionMapper {
+	static fromEntity(entity: RepositoryVersionEntity): RepositoryVersion {
+		return new RepositoryVersion({
+			id: entity.id,
+			repositoryId: entity.repository_id,
+			tag: entity.tag,
+			title: entity.title,
+			state: entity.state,
+			totalSnippets: entity.total_snippets ?? 0,
+			indexedAt: entity.indexed_at != null ? new Date(entity.indexed_at * 1000) : null,
+			createdAt: new Date(entity.created_at * 1000)
+		});
+	}
+
+	static toDto(domain: RepositoryVersion): RepositoryVersionDto {
+		return new RepositoryVersionDto({
+			id: domain.id,
+			repositoryId: domain.repositoryId,
+			tag: domain.tag,
+			title: domain.title,
+			state: domain.state,
+			totalSnippets: domain.totalSnippets,
+			indexedAt: domain.indexedAt,
+			createdAt: domain.createdAt
+		});
+	}
+}