chore: open-source contribution workflow — tests, CI, contributing guide

[shaulams]

What does this PR do?

Sets up a proper open-source contribution workflow for FieldCut:

1. Test Suite (52 tests)

• test_helpers.py — merge_segments, friendly_error, get_clip_speaker logic
• test_routes.py — all Flask endpoints: smoke tests, input validation, clip CRUD, project management, speakers, metadata, audio endpoints
• test_pipeline.py — end-to-end: mark clips → cut with ffmpeg → verify output files; project save/load/duplicate/delete lifecycle; waveform extraction
• No API keys required — external services are mocked/skipped

2. GitHub Actions CI

⚠️ Needs manual addition — the OAuth token doesn't have workflow scope. Please create .github/workflows/ci.yml manually (content below) or re-push with a PAT that has workflow permissions.

name: CI
on:
  pull_request:
    branches: [main]
  push:
    branches: [main]
jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-python@v5
        with:
          python-version: "3.11"
      - run: sudo apt-get update && sudo apt-get install -y ffmpeg
      - run: pip install -r requirements.txt && pip install pytest
      - run: pytest tests/ -v --tb=short

3. CONTRIBUTING.md

Clear instructions for contributors: fork, setup, run tests, branch naming, commit conventions, PR guidelines.

4. PR Template

Consistent format with checklist: change type, testing steps, screenshots for UI changes.

5. Branch Protection (manual)

After merging, enable branch protection on main:

• Settings → Branches → Add rule for main
• ✅ Require status checks to pass (select the CI job)
• ✅ Require pull request reviews

Type of change

• New feature (contribution infrastructure)

How to test

pip install pytest
pytest tests/ -v

Checklist

• Tested locally (52/52 passing)
• Tests included
• Documentation updated (CONTRIBUTING.md)