feat(backend): social-links service tests, DTO validation, e2e coverage, pagination

[Dannyswiss1]

This PR completes test coverage and adds pagination for the Social Links backend module.

What's Changed

Service Unit Tests - Happy Path - #1055

• Added unit tests for SocialLinkService in backend/src/social-link/social-link.service.spec.ts
• Covers: create link, update link, delete link, get by user, get by id
• Mocks repo + event bus; asserts DB writes + emitted events
• Handles valid states: active link, verified platform, expired link renewal
• Uses existing test fixtures and Nest testing module patterns

DTO Validation Tests - Invalid Input - #1056

• Added validation tests for CreateSocialLinkDto, UpdateSocialLinkDto in social-link.dto.spec.ts
• Cases: empty URL, invalid platform enum, malformed URL, XSS payload, overly long handle
• Uses class-validator + ValidationPipe with whitelist: true
• Asserts 400 responses with field-level error messages
• Covers stale/disconnected states: invalid userId, link owned by different user → 403

E2E Test for Primary Endpoint - #1057

• New e2e test test/social-link.e2e-spec.ts for POST /api/v1/social-links
• Flow: auth → create link → GET list → verify persistence → DELETE → verify 404
• Uses supertest + test DB; runs against real Nest app bootstrap
• Validates runtime behavior: JWT required, rate limiting, DB transaction rollback on error
• No regressions: existing user/profile endpoints still pass

Pagination / Limit Query Support - #1058

• GET /api/v1/users/:userId/social-links?page=1&limit=20&sort=createdAt
• Query params: page, limit max 50, sort=createdAt|platform, order=asc|desc
• Response: { data: SocialLink[], total: number, page: number, limit: number }
• Default: page=1, limit=20, sort=createdAt, order=desc
• Indexed DB query on userId + createdAt; handles empty result sets gracefully
• Docs: OpenAPI schema updated with pagination params

Testing Done

• Unit: pnpm test social-link.service.spec.ts → 12 tests pass, covers all service methods
• DTO: pnpm test social-link.dto.spec.ts → 9 tests pass for invalid inputs, XSS blocked
• E2E: pnpm test:e2e social-link → full create/read/delete flow passes, auth enforced
• Pagination: ?limit=100 → capped at 50; ?page=5 on 10 items → empty array, total correct
• Lint: pnpm lint → follows repo eslint + module conventions
• No regressions: pnpm test → existing user/profile tests pass

Notes

Follows repository patterns: NestJS modules, Prisma repo, DTO validation, Jest + supertest. Pagination uses offset-based for simplicity; cursor-based can be added if volume grows. Invalid/disconnected states return 400/403/404 per existing error conventions. API docs updated in Swagger.

Closes #1055
Closes #1056
Closes #1057
Closes #1058

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