[PROPOSAL] Add versioned REST API endpoints for test results for NG frontend

## [PROPOSAL] Add versioned REST API endpoints for test results to support Sample Platform NG frontend

### Problem
The current platform primarily serves HTML pages and has very limited API surface. The NG frontend direction needs stable, versioned JSON endpoints for test summary/results/files/progress and category metadata.

### Proposed endpoints
- `GET /api/v1/tests//summary`
- `GET /api/v1/tests//results`
- `GET /api/v1/tests//files`
- `GET /api/v1/tests//progress`
- `GET /api/v1/categories`

### Proposed implementation notes
- Introduce a dedicated `mod_api` blueprint.
- Reuse existing model/controller logic where possible to avoid behavior drift.
- Return structured JSON with consistent status envelope and 404 handling for missing tests.
- Add focused API tests for success and not-found paths.

### Why this is valuable
- Unblocks frontend/API decoupling for NG work.
- Creates a clear contract for external consumers and automation.
- Reduces coupling to server-rendered templates.

### Verification plan
- Add integration tests under `tests/test_api/`.
- Validate endpoint behavior locally with `nose2` test suite.
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- [X] I have checked that the bug-fix I am reporting can be replicated, or that the feature I am suggesting isn't already present.
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