test: add CI test coverage for core workflows

[chestercheng]

Changes

• Add locked GitHub Actions checks for Ruff lint, Ruff formatting, pytest, and a downloadable coverage report.
• Add an OIDC login scenario using a local provider:
  • chester@example.com is allowed.
  • steve@example.com is denied.
• Add a report delivery scenario:
  • Seed an in-memory database with an active event and tickets.
  • Run report.send_report().
  • Verify the rendered Discord payload and delivery timestamp behavior.
  • Verify failed Discord delivery leaves the report pending for retry.
• Add a saved Sciwork KKTIX page fragment for parser coverage.
• Verify webhook logs redact authorization credentials and contact PII.
• Move Discord payload text and schema into the packaged report.json.j2 production template.

Verification

• uv run --locked --group dev ruff check src tests
• uv run --locked --group dev ruff format --check src tests
• uv run --locked --group dev pytest tests/ -W error --cov=argus --cov-report=term-missing --cov-report=xml
• uv lock --check

[codecov]

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