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.test.ts

@@ -458,6 +458,31 @@ describe('IndexingPipeline', () => {
 		expect(updated.progress).toBe(100);
 	});
 
+	it('uses the repository source_url when crawling local repositories', async () => {
+		const crawl = vi.fn().mockResolvedValue({
+			files: [],
+			totalFiles: 0,
+			skippedFiles: 0,
+			branch: 'local',
+			commitSha: 'abc'
+		});
+
+		const pipeline = new IndexingPipeline(
+			db,
+			vi.fn() as never,
+			{ crawl } as never,
+			null
+		);
+
+		const job = makeJob();
+		await pipeline.run(job as never);
+
+		expect(crawl).toHaveBeenCalledWith({
+			rootPath: '/tmp/test-repo',
+			ref: undefined
+		});
+	});
+
 	it('integration: handles unchanged, modified, added, and deleted files in one run', async () => {
 		// ---- First run: index three files -----------------------------------
 		const firstFiles = [