Backend: add API integration test suite with fixtures and coverage

[nasalehj]

Summary

Resolves #756. Adds backend/test/api-integration.e2e-spec.ts to the e2e run, with reusable DB fixtures, live route discovery, and coverage reporting in CI.

Automated coverage of all public API endpoints

• The suite reads the application's own Swagger document at test time and calls every mounted operation (method + path). That list cannot drift from the code, and a framework Cannot <METHOD> 404 fails the suite, proving the route is mounted.
• Discovery is asserted to include the core routes (/puzzles/active, /puzzle-submission, /health/live) and to yield more than 50 operations.

Test DB fixtures

• backend/test/fixtures/test-database.fixture.ts provides resetDatabase() (truncates every entity table with RESTART IDENTITY CASCADE) and seedDatabase() (deterministic admin, player, active/inactive puzzles, completion, reward, category, category puzzle, content, and progress).
• Fixture-backed flows cover: puzzle discovery hiding solutions/hints, admin puzzle creation → grading (correct + wrong, case-insensitive) → publication, anonymous admin writes returning 401, reward creation/listing and duplicate-claim 409, and seeded content/categories.

Runs in CI + reports coverage

• New npm run test:e2e:cov writes text-summary + lcov to coverage-e2e/.
• The backend-e2e job now runs with coverage and uploads backend-e2e-coverage as an artifact.

Supporting fix

• Progress.userId declared type: 'uuid' with length: 128, which makes TypeORM throw Column userId of Entity Progress does not support length property when building metadata outside the full app. Removed the invalid length.

Verification

• The fixture layer was executed against a real PostgreSQL 16 instance: seedDatabase() inserted 1 admin, 1 user, 2 puzzles, 1 reward, and 1 content row; resetDatabase() then left every table empty.
• tsc --noEmit reports no errors for the new files.
• Limitation: the repository's Jest e2e runner currently cannot load its own NestJS 12 ESM packages (SyntaxError: Unexpected token 'export' from @nestjs/schedule/@nestjs/typeorm), which also blocks the pre-existing app.e2e-spec.ts from running locally. The new spec is subject to the same pre-existing runner configuration; the suite itself is wired into the existing backend-e2e CI job and will run wherever that job runs.

Closes #756

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