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/models/repository-version.ts

@@ -0,0 +1,98 @@
+export interface RepositoryVersionEntityProps {
+	id: string;
+	repository_id: string;
+	tag: string;
+	title: string | null;
+	state: 'pending' | 'indexing' | 'indexed' | 'error';
+	total_snippets: number | null;
+	indexed_at: number | null;
+	created_at: number;
+}
+
+export class RepositoryVersionEntity {
+	id: string;
+	repository_id: string;
+	tag: string;
+	title: string | null;
+	state: 'pending' | 'indexing' | 'indexed' | 'error';
+	total_snippets: number | null;
+	indexed_at: number | null;
+	created_at: number;
+
+	constructor(props: RepositoryVersionEntityProps) {
+		this.id = props.id;
+		this.repository_id = props.repository_id;
+		this.tag = props.tag;
+		this.title = props.title;
+		this.state = props.state;
+		this.total_snippets = props.total_snippets;
+		this.indexed_at = props.indexed_at;
+		this.created_at = props.created_at;
+	}
+}
+
+export interface RepositoryVersionProps {
+	id: string;
+	repositoryId: string;
+	tag: string;
+	title: string | null;
+	state: 'pending' | 'indexing' | 'indexed' | 'error';
+	totalSnippets: number;
+	indexedAt: Date | null;
+	createdAt: Date;
+}
+
+export class RepositoryVersion {
+	id: string;
+	repositoryId: string;
+	tag: string;
+	title: string | null;
+	state: 'pending' | 'indexing' | 'indexed' | 'error';
+	totalSnippets: number;
+	indexedAt: Date | null;
+	createdAt: Date;
+
+	constructor(props: RepositoryVersionProps) {
+		this.id = props.id;
+		this.repositoryId = props.repositoryId;
+		this.tag = props.tag;
+		this.title = props.title;
+		this.state = props.state;
+		this.totalSnippets = props.totalSnippets;
+		this.indexedAt = props.indexedAt;
+		this.createdAt = props.createdAt;
+	}
+}
+
+export interface RepositoryVersionDtoProps {
+	id: string;
+	repositoryId: string;
+	tag: string;
+	title: string | null;
+	state: 'pending' | 'indexing' | 'indexed' | 'error';
+	totalSnippets: number;
+	indexedAt: Date | null;
+	createdAt: Date;
+}
+
+export class RepositoryVersionDto {
+	id: string;
+	repositoryId: string;
+	tag: string;
+	title: string | null;
+	state: 'pending' | 'indexing' | 'indexed' | 'error';
+	totalSnippets: number;
+	indexedAt: Date | null;
+	createdAt: Date;
+
+	constructor(props: RepositoryVersionDtoProps) {
+		this.id = props.id;
+		this.repositoryId = props.repositoryId;
+		this.tag = props.tag;
+		this.title = props.title;
+		this.state = props.state;
+		this.totalSnippets = props.totalSnippets;
+		this.indexedAt = props.indexedAt;
+		this.createdAt = props.createdAt;
+	}
+}