feat(interview): add cancel interview enpoint

[Bnabdulwasiu]

Description

This PR implements the "Cancel Interview" endpoint (POST /api/v1/interviews/:id/cancel) as specified in the Interviews Core CRUD feature set. It allows recruiters to cancel a scheduled or draft interview session, transitions its status to cancelled, and prevents already-cancelled or completed interviews from being modified.

Type of Change

• feat — New feature
• fix — Bug fix
• refactor — Code refactoring (no functional change)
• docs — Documentation update
• test — Adding or updating tests
• chore — Maintenance (dependencies, CI, tooling)

Related Issue

Closes #

Changes Made

• Service Layer (app/services/interview.py): Added cancel_interview method to manage the cancelled state transitions and prevent operations on completed/cancelled interviews (raising 409 Conflict).
• Route Layer (app/api/v1/routes/interviews.py): Defined the POST /api/v1/interviews/{interview_id}/cancel route ensuring secure authorization (only owners/interviewers can cancel their own interviews).
• Test Suite (tests/test_cancel_interview.py): Added comprehensive test cases verifying happy path cancellation, state transitions, authentication (401), query safety (404), cross-user leak protection (404), and double-cancellation conflicts (409).

Proof of Work

API Response / Screenshots

// POST /api/v1/interviews/a905c9f.../cancel
// Status: 200 OK
{
  "status": "success",
  "message": "Interview cancelled successfully",
  "data": {
    "id": "a905c9f2-...",
    "title": "Senior Backend Engineer",
    "status": "cancelled",
    "role_title": "Senior Backend Engineer",
    "platform": "zoom",
    "ai_tone": "professional",
    "candidate_name": "Jane Doe",
    "candidate_email": "jane@example.com",
    "summary": {
      "job_description": "Build scalable APIs using FastAPI and PostgreSQL.",
      "scoring_rubric": "Communication, API design, problem-solving, scalability.",
      "ai_assessment": null,
      "status": "pending"
    },
    "criteria": [
      "Communication",
      "API Design",
      "Problem Solving"
    ],
    "created_at": "2026-05-19T14:52:00Z"
  }
}

## Test Cases

- [x] Test case 1: `test_cancel_draft_interview_returns_200`
- [x] Test case 2: `test_cancel_returns_correct_interview_fields`
- [x] Test case 3: `test_cancel_is_reflected_in_get`
- [x] Test case 4: `test_cancel_returns_404_for_nonexistent_interview`
- [x] Test case 5: `test_cancel_returns_404_for_another_users_interview`
- [x] Test case 6: `test_cancel_returns_401_without_token`
- [x] Test case 7: `test_cancel_already_cancelled_returns_409`

<details>
<summary>Test output</summary>

```bash
$ uv run pytest tests/test_cancel_interview.py -v
============================= test session starts =============================
platform win32 -- Python 3.13.13, pytest-9.0.3, pluggy-1.6.0
cachedir: .pytest_cache
rootdir: C:\Users\USER\Desktop\meetmind-api
configfile: pyproject.toml
plugins: anyio-4.13.0, asyncio-1.3.0
asyncio: mode=Mode.AUTO, debug=False
collected 7 items

tests/test_cancel_interview.py::TestCancelInterview::test_cancel_draft_interview_returns_200 PASSED [ 14%]
tests/test_cancel_interview.py::TestCancelInterview::test_cancel_returns_correct_interview_fields PASSED [ 28%]
tests/test_cancel_interview.py::TestCancelInterview::test_cancel_is_reflected_in_get PASSED [ 42%]
tests/test_cancel_interview.py::TestCancelInterview::test_cancel_returns_404_for_nonexistent_interview PASSED [ 57%]
tests/test_cancel_interview.py::TestCancelInterview::test_cancel_returns_404_for_another_users_interview PASSED [ 71%]
tests/test_cancel_interview.py::TestCancelInterview::test_cancel_returns_401_without_token PASSED [ 85%]
tests/test_cancel_interview.py::TestCancelInterview::test_cancel_already_cancelled_returns_409 PASSED [100%]

============================== 7 passed in 8.73s ==============================


## Checklist

- [x] My branch follows the naming convention (`<type>/<short-description>`)
- [x] My commits follow [Conventional Commits](https://www.conventionalcommits.org/)
- [x] I have added meaningful tests that cover success and failure paths
- [x] All new and existing tests pass locally (`uv run pytest`)
- [x] I have included proof of work (JSON responses or screenshots)
- [ ] I have updated documentation if needed
- [x] My code follows the project's style guidelines