Add waitlist position cap option to signup queue items

[nbudin]

Summary

• Adds an optional waitlist_position_cap field to SignupRankedChoice that limits waitlist prioritization to only apply when the attendee would be at or below a specified position (1–10)
• When the cap would be exceeded, the choice is skipped with a new WAITLIST_POSITION_CAP_EXCEEDED reason
• Frontend shows a dropdown (1–10 or "regardless of position") beneath the "waitlist instead of skipping" checkbox

Fixes #11223

Test plan

• Enable "waitlist instead of skipping" on a queue item and verify the cap dropdown appears
• Select a cap value and verify it is saved and persists
• Select "regardless of position" and verify the cap is cleared (null)
• In a full event scenario, verify the simulated skip reason shows "waitlist position cap exceeded" when the cap would be exceeded
• Verify the signup round automation skips with waitlist_position_cap_exceeded when applicable

🤖 Generated with Claude Code

[github-actions]

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
app/graphql/mutations/set_signup_ranked_choice_prioritize_waitlist.rb	🟢 80%	🟠 69.23%	🔴 -10.77%
app/graphql/types/signup_ranked_choice_type.rb	🟢 96.77%	🟢 96.97%	🟢 0.2%
Overall Coverage	🟢 52.65%	🟢 52.69%	🟢 0.04%

Minimum allowed coverage is 0%, this run produced 52.69%