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/search-result.mapper.ts

@@ -0,0 +1,35 @@
+import { LibrarySearchResult, SnippetRepositoryRef, SnippetSearchResult } from '$lib/server/models/search-result.js';
+import { RepositoryMapper } from '$lib/server/mappers/repository.mapper.js';
+import { RepositoryVersionMapper } from '$lib/server/mappers/repository-version.mapper.js';
+import { SnippetMapper } from '$lib/server/mappers/snippet.mapper.js';
+import { RepositoryEntity } from '$lib/server/models/repository.js';
+import { RepositoryVersionEntity } from '$lib/server/models/repository-version.js';
+import { SnippetEntity } from '$lib/server/models/snippet.js';
+
+export class SearchResultMapper {
+	static snippetFromEntity(
+		entity: SnippetEntity,
+		repository: { id: string; title: string },
+		score: number
+	): SnippetSearchResult {
+		return new SnippetSearchResult({
+			snippet: SnippetMapper.fromEntity(entity),
+			score,
+			repository: new SnippetRepositoryRef(repository)
+		});
+	}
+
+	static libraryFromEntity(
+		repositoryEntity: RepositoryEntity,
+		versionEntities: RepositoryVersionEntity[],
+		score: number
+	): LibrarySearchResult {
+		return new LibrarySearchResult({
+			repository: RepositoryMapper.fromEntity(repositoryEntity),
+			versions: versionEntities.map((version) =>
+				RepositoryVersionMapper.fromEntity(version)
+			),
+			score
+		});
+	}
+}