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/services/repository.service.test.ts

@@ -281,9 +281,9 @@ describe('RepositoryService.add()', () => {
 		const repo = service.add({
 			source: 'github',
 			sourceUrl: 'https://github.com/facebook/react'
-		}) as unknown as RawRepo;
-		expect(repo.total_snippets).toBe(0);
-		expect(repo.total_tokens).toBe(0);
+		});
+		expect(repo.totalSnippets).toBe(0);
+		expect(repo.totalTokens).toBe(0);
 	});
 });
 
@@ -443,9 +443,9 @@ describe('RepositoryService.createIndexingJob()', () => {
 	});
 
 	it('creates a queued indexing job', () => {
-		const job = service.createIndexingJob('/facebook/react') as unknown as RawJob;
+		const job = service.createIndexingJob('/facebook/react');
 		expect(job.id).toBeTruthy();
-		expect(job.repository_id).toBe('/facebook/react');
+		expect(job.repositoryId).toBe('/facebook/react');
 		expect(job.status).toBe('queued');
 		expect(job.progress).toBe(0);
 	});
@@ -467,10 +467,7 @@ describe('RepositoryService.createIndexingJob()', () => {
 	});
 
 	it('accepts an optional versionId', () => {
-		const job = service.createIndexingJob(
-			'/facebook/react',
-			'/facebook/react/v18.3.0'
-		) as unknown as RawJob;
-		expect(job.version_id).toBe('/facebook/react/v18.3.0');
+		const job = service.createIndexingJob('/facebook/react', '/facebook/react/v18.3.0');
+		expect(job.versionId).toBe('/facebook/react/v18.3.0');
 	});
 });