Add rerandomize_lottery_numbers option to SignupRound

[nbudin]

Summary

• Adds a rerandomize_lottery_numbers boolean field to SignupRound (default false)
• When true, the automation reassigns new random lottery numbers (sampled from 1–10,000 without replacement) to all UserConProfiles in the convention before executing the round's signups, using a single UPDATE … FROM (VALUES …) SQL statement to avoid unique constraint issues
• New numbers are assigned atomically within the same transaction as the signup execution, ensuring they apply to the round that's about to run

Test plan

• Added test verifying rerandomized lottery numbers are in the 1–10,000 range and are used for signup ordering
• Added test verifying lottery numbers are unchanged when rerandomize_lottery_numbers is false
• All existing ExecuteRankedChoiceSignupRoundService tests continue to pass

Fixes #11224

🤖 Generated with Claude Code

[github-actions]

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
app/graphql/types/signup_round_type.rb	🟢 95%	🟢 95.45%	🟢 0.45%
app/services/execute_ranked_choice_signup_round_service.rb	🟢 80.39%	🟢 81.65%	🟢 1.26%
test/services/execute_ranked_choice_signup_round_service_test.rb	🟢 94.58%	🟢 95.09%	🟢 0.51%
Overall Coverage	🟢 52.69%	🟢 52.75%	🟢 0.06%

Minimum allowed coverage is 0%, this run produced 52.75%