Give the registration policy editor real bucket ids (phase 1 of dropping RegistrationPolicyBucket#key)

[nbudin]

Background

#11870 (merged in #11891) and #11892 (merged in #11893) removed RegistrationPolicyBucket#key as the identity mechanism used internally by signup simulation, occupancy checks, and editing correlation. What's left is external: the browser-based registration policy editor (RegistrationPolicyEditor/RegistrationBucketRow, rendered via RegistrationPolicyItemInput on the event edit form) never sends bucket id at all, for either existing or new buckets.

This isn't just an internal-cleanup gap -- it's the one remaining reason RegistrationPolicyBucket#key still needs to be a real, persisted, unique database column:

• RegistrationPolicy#sync_buckets_from_hash! already prefers matching an incoming bucket hash by id, falling back to key only when no id is supplied (app/models/registration_policy.rb). Every edit made through the current UI takes that fallback path, because the UI never supplies an id.
• RegistrationPolicy#build_from_hash and IGNORED_BUCKET_HASH_KEYS already keep a bucket's id if the incoming hash has one -- this was built for exactly this purpose in Correlate registration policy buckets by id instead of key when editing/simulating policy changes #11870, but nothing has fed it real data yet.
• The reason the browser never sees a bucket's id in the first place: RegistrationPolicyBucket#as_json (used to populate form_response_attrs_json, the only way the edit form receives registration policy data) deliberately omits id, and the frontend's EditingRegistrationBucket type (app/javascript/RegistrationPolicy/RegistrationBucketRow.tsx) explicitly excludes id from its editing state.

Separately, RegistrationPolicyBucketType#key (the bucket's own read-only GraphQL field, app/graphql/types/registration_policy_bucket_type.rb) is queried and used as a business-logic lookup key (not just displayed) in several other frontend query paths, unrelated to the editor:

• app/javascript/EventsApp/ScheduleGrid/queries.graphql -> AvailabilityUtils.tsx
• app/javascript/EventsApp/EventPage/queries.graphql -> buildSignupOptions.ts, EventPageRunCard.tsx, CreateModeratedSignupModal.tsx, RunCapacityGraphBucket.tsx, EventCapacityDisplay.tsx, RunCapacityGraph.tsx
• app/javascript/EventsApp/MySignupQueue/queries.graphql -> UserSignupQueueItem.tsx
• app/javascript/EventsApp/SignupAdmin/queries.graphql -> SignupUtils.ts (findBucket), BucketInput.tsx, route.tsx, RunSignupsTable.tsx, RunEmailList.tsx
• app/javascript/SignupModeration/queries.graphql -> CreateSignupRunCard.tsx, SignupModerationQueue.tsx
• app/javascript/EventAdmin/queries.graphql -> BucketKeyRemappingModal.tsx, useBucketKeyRemapping.tsx
• app/javascript/Reports/queries.graphql -> Tables/BucketChangeCell.tsx (via SignupUtils.ts)

Almost all of these are find/lookup/Object.fromEntries/React key= prop usages, directly convertible to bucket.id. A few (RunSignupsTable.tsx, RunEmailList.tsx, BucketKeyRemappingModal.tsx) fall back to bucket.name ?? bucket.key for display text -- since name is a NOT NULL database column, that fallback branch is already dead code and can just become bucket.name.

Proposed Change

1. RegistrationPolicyBucket#as_json (app/models/registration_policy_bucket.rb): add "id" => id to the hash. This also changes the shape of future FormResponseChange audit records (additive only -- existing readers that only look at key/name/etc. are unaffected).
2. RegistrationPolicyEditor.tsx/RegistrationBucketRow.tsx/app/javascript/RegistrationPolicy/RegistrationPolicy.ts: extend EditingRegistrationBucket (and the add/update/remove helpers) to carry an optional id -- undefined for a bucket added in the current edit session, the real id for anything loaded from the server. Thread it through into the hash sent back as form_response_attrs_json.
3. Convert the business-logic lookups in the file list above from bucket.key to bucket.id. Fix the bucket.name ?? bucket.key display fallbacks to just bucket.name.

Benefits

• Makes every edit through the registration policy editor correlate buckets by id, matching what Correlate registration policy buckets by id instead of key when editing/simulating policy changes #11870/Convert remaining registration policy bucket key dependencies (SignupBucketFinder, EventChangeRegistrationPolicyService) to id #11892 already assume is possible.
• Removes the last reason sync_buckets_from_hash!'s key-based fallback is load-bearing for the live updateEvent mutation.
• Clears the way for Deprecate and remove the remaining bucket key-based GraphQL/Liquid surface (phase 2 of dropping RegistrationPolicyBucket#key) #11896 (deprecate/remove the key-based GraphQL surface), Retire RegistrationPolicyBucket#key as an identity mechanism in the app layer (phase 3 of dropping RegistrationPolicyBucket#key) #11897 (retire key as identity in the app layer), and Drop the registration_policy_buckets.key column (phase 4 of dropping RegistrationPolicyBucket#key) #11898 (drop the column).

Tradeoffs

• Changes the shape of future FormResponseChange audit JSON (adds an id key) -- additive, but worth calling out since a prior comment on RegistrationPolicy#as_json was explicit about protecting that shape.
• This is a real frontend state-management change (not just a mechanical rename) across several components; should be reviewed carefully for regressions in the registration policy editor, bucket key remapping modal, and signup admin bucket pickers.

References

• Parent: Convert bucket_key/requested_bucket_key to real foreign keys #11868
• Predecessors: Correlate registration policy buckets by id instead of key when editing/simulating policy changes #11870 (merged in Correlate registration policy buckets by id instead of key when editing #11891), Convert remaining registration policy bucket key dependencies (SignupBucketFinder, EventChangeRegistrationPolicyService) to id #11892 (merged in Convert remaining registration policy bucket key dependencies to id #11893)
• Part of a 4-phase plan to eventually drop registration_policy_buckets.key; see Deprecate and remove the remaining bucket key-based GraphQL/Liquid surface (phase 2 of dropping RegistrationPolicyBucket#key) #11896, Retire RegistrationPolicyBucket#key as an identity mechanism in the app layer (phase 3 of dropping RegistrationPolicyBucket#key) #11897, Drop the registration_policy_buckets.key column (phase 4 of dropping RegistrationPolicyBucket#key) #11898

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Issue drafted by Claude