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/routes/api/v1/settings/embedding/test/+server.ts

@@ -14,6 +14,16 @@ import {
 } from '$lib/server/embeddings/factory';
 import { handleServiceError, InvalidInputError } from '$lib/server/utils/validation';
 
+export const GET: RequestHandler = async () => {
+	try {
+		const provider = createProviderFromConfig({ provider: 'local' });
+		const available = await provider.isAvailable();
+		return json({ available });
+	} catch (err) {
+		return handleServiceError(err);
+	}
+};
+
 // ---------------------------------------------------------------------------
 // Validate — reuse the same shape accepted by PUT /settings/embedding
 // ---------------------------------------------------------------------------