Add E2E tests for CfP process

[bouge]

In advance of any changes to CfP -> Content as per issue #1868 - adding E2E testing to CfP to help track any regressions with future changes.

Adds comprehensive end-to-end tests covering the complete CfP lifecycle:

• Proposal submission - All 6 types via web forms
• State transitions - Full review workflow from new > finalised
• Scheduling - CLI commands (schedule, apply_potential_schedule)
• Favouriting - Add/remove via web routes, schedule visibility
• Clash detection - overlaps_with(), get_conflicting_content(), ClashFinder tool

Adds CLI wrapper script and additional test container for validation

There is a PR open in the slotmachine repo emfcamp/slotmachine#10 to fix depreciated parameters after a version bump of PuLP.

To test this PR prior to that being merged you can updated pyproject.toml on line 101 to use the forked version from which the PR is raised.

slotmachine = { git = "https://github.com/bouge/slotmachine.git" }

[bouge]

That's my mistake, using default PyCharm linting, will fix