Migrate registration policies from JSONB to database tables

[nbudin]

Background

Registration policies (and their buckets) are currently stored as JSONB objects on the events table. This makes it impossible to enforce referential integrity on fields like bucket_key and requested_bucket_key in signups, signup_requests, and signup_ranked_choices — they're just free-text strings that reference a key in a JSON blob.

This has caused real problems: #11229 was filed because changes to bucket keys could silently invalidate those references, and we had to add application-level logic to detect and remap them.

Proposed Change

Extract registration policies and their buckets into proper database tables:

• registration_policies table (belongs to event)
• registration_policy_buckets table (belongs to registration_policy, has key, name, description, slots_limited, total_slots, minimum_slots, preferred_slots, anything, not_counted, etc.)

Then migrate bucket_key and requested_bucket_key on signups, signup_requests, and signup_ranked_choices to be integer foreign key columns referencing registration_policy_buckets.

Benefits

• Referential integrity enforced at the database level — no more stale bucket key strings
• Renaming a bucket updates all references automatically (or fails with a FK violation)
• Simplifies the validation and remapping logic added in Changes to bucket keys can invalidate signups, signup requests, and queue items #11229
• Opens the door to querying signup counts per bucket more efficiently via SQL joins

Considerations

• This is a significant migration — the JSONB serialization is used throughout the codebase (GraphQL types, forms, services)
• The RegistrationPolicy and RegistrationPolicy::Bucket value objects would need to either be replaced or backed by ActiveRecord models
• The EventChangeRegistrationPolicyService logic would need to be revisited
• Liquid drops and other serialization points would need updating