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/mappers/context-response.mapper.ts

@@ -0,0 +1,71 @@
+import {
+	CodeListItemDto,
+	CodeSnippetJsonDto,
+	ContextJsonResponseDto,
+	InfoSnippetJsonDto,
+	LibrarySearchJsonResponseDto,
+	LibrarySearchJsonResultDto,
+	type SnippetJsonDto
+} from '$lib/server/models/context-response.js';
+import { LibrarySearchResult, SnippetSearchResult } from '$lib/server/models/search-result.js';
+
+export class ContextResponseMapper {
+	static toLibrarySearchJson(results: LibrarySearchResult[]): LibrarySearchJsonResponseDto {
+		return new LibrarySearchJsonResponseDto(
+			results.map(
+				({ repository, versions }) =>
+					new LibrarySearchJsonResultDto({
+						id: repository.id,
+						title: repository.title,
+						description: repository.description ?? null,
+						branch: repository.branch ?? null,
+						lastUpdateDate: repository.lastIndexedAt
+							? repository.lastIndexedAt.toISOString()
+							: null,
+						state: repository.state === 'indexed' ? 'finalized' : repository.state === 'error' ? 'error' : 'initial',
+						totalTokens: repository.totalTokens ?? null,
+						totalSnippets: repository.totalSnippets ?? null,
+						stars: repository.stars ?? null,
+						trustScore: repository.trustScore ?? null,
+						benchmarkScore: repository.benchmarkScore ?? null,
+						versions: versions.map((version) => version.tag),
+						source: repository.sourceUrl
+					})
+			)
+		);
+	}
+
+	static toContextJson(snippets: SnippetSearchResult[], rules: string[]): ContextJsonResponseDto {
+		const mapped: SnippetJsonDto[] = snippets.map(({ snippet }) => {
+			if (snippet.type === 'code') {
+				return new CodeSnippetJsonDto({
+					title: snippet.title ?? null,
+					description: snippet.breadcrumb ?? null,
+					language: snippet.language ?? null,
+					codeList: [
+						new CodeListItemDto({
+							language: snippet.language ?? '',
+							code: snippet.content
+						})
+					],
+					id: snippet.id,
+					tokenCount: snippet.tokenCount ?? null,
+					pageTitle: snippet.breadcrumb ? snippet.breadcrumb.split('>')[0].trim() || null : null
+				});
+			}
+
+			return new InfoSnippetJsonDto({
+				text: snippet.content,
+				breadcrumb: snippet.breadcrumb ?? null,
+				pageId: snippet.id,
+				tokenCount: snippet.tokenCount ?? null
+			});
+		});
+
+		return new ContextJsonResponseDto({
+			snippets: mapped,
+			rules,
+			totalTokens: snippets.reduce((sum, result) => sum + (result.snippet.tokenCount ?? 0), 0)
+		});
+	}
+}