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/context-response.ts

@@ -0,0 +1,99 @@
+export class LibrarySearchJsonResultDto {
+	id: string;
+	title: string;
+	description: string | null;
+	branch: string | null;
+	lastUpdateDate: string | null;
+	state: 'initial' | 'finalized' | 'error';
+	totalTokens: number | null;
+	totalSnippets: number | null;
+	stars: number | null;
+	trustScore: number | null;
+	benchmarkScore: number | null;
+	versions: string[];
+	source: string;
+
+	constructor(props: LibrarySearchJsonResultDto) {
+		this.id = props.id;
+		this.title = props.title;
+		this.description = props.description;
+		this.branch = props.branch;
+		this.lastUpdateDate = props.lastUpdateDate;
+		this.state = props.state;
+		this.totalTokens = props.totalTokens;
+		this.totalSnippets = props.totalSnippets;
+		this.stars = props.stars;
+		this.trustScore = props.trustScore;
+		this.benchmarkScore = props.benchmarkScore;
+		this.versions = props.versions;
+		this.source = props.source;
+	}
+}
+
+export class LibrarySearchJsonResponseDto {
+	results: LibrarySearchJsonResultDto[];
+
+	constructor(results: LibrarySearchJsonResultDto[]) {
+		this.results = results;
+	}
+}
+
+export class CodeListItemDto {
+	language: string;
+	code: string;
+
+	constructor(props: CodeListItemDto) {
+		this.language = props.language;
+		this.code = props.code;
+	}
+}
+
+export class CodeSnippetJsonDto {
+	type: 'code' = 'code';
+	title: string | null;
+	description: string | null;
+	language: string | null;
+	codeList: CodeListItemDto[];
+	id: string;
+	tokenCount: number | null;
+	pageTitle: string | null;
+
+	constructor(props: Omit<CodeSnippetJsonDto, 'type'>) {
+		this.title = props.title;
+		this.description = props.description;
+		this.language = props.language;
+		this.codeList = props.codeList;
+		this.id = props.id;
+		this.tokenCount = props.tokenCount;
+		this.pageTitle = props.pageTitle;
+	}
+}
+
+export class InfoSnippetJsonDto {
+	type: 'info' = 'info';
+	text: string;
+	breadcrumb: string | null;
+	pageId: string;
+	tokenCount: number | null;
+
+	constructor(props: Omit<InfoSnippetJsonDto, 'type'>) {
+		this.text = props.text;
+		this.breadcrumb = props.breadcrumb;
+		this.pageId = props.pageId;
+		this.tokenCount = props.tokenCount;
+	}
+}
+
+export type SnippetJsonDto = CodeSnippetJsonDto | InfoSnippetJsonDto;
+
+export class ContextJsonResponseDto {
+	snippets: SnippetJsonDto[];
+	rules: string[];
+	totalTokens: number;
+
+	constructor(props: ContextJsonResponseDto) {
+		this.snippets = props.snippets;
+		this.rules = props.rules;
+		this.totalTokens = props.totalTokens;
+	}
+}