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>

.claude/settings.local.json

@@ -1,25 +0,0 @@
-{
-  "permissions": {
-    "allow": [
-      "WebFetch(domain:raw.githubusercontent.com)",
-      "WebFetch(domain:api.github.com)",
-      "WebFetch(domain:github.com)",
-      "Bash(git init:*)",
-      "Bash(git add:*)",
-      "Bash(git commit:*)",
-      "Bash(git checkout:*)",
-      "Bash(DATABASE_URL=./local.db npx drizzle-kit generate 2>&1)",
-      "Bash(DATABASE_URL=./local.db npx drizzle-kit generate)",
-      "Bash(npm run:*)",
-      "Bash(npm test:*)",
-      "Skill(update-config)",
-      "Bash(git -C /home/moze/Sources/trueref checkout -b feat/TRUEREF-0002-through-0018)",
-      "Bash(git:*)"
-    ],
-    "defaultMode": "bypassPermissions"
-  },
-  "enableAllProjectMcpServers": true,
-  "enabledMcpjsonServers": [
-    "svelte"
-  ]
-}