Fix N+1 queries on registration_policy and its buckets

[nbudin]

Summary

• EventType/EventProposalType read registration_policy directly, and RegistrationPolicyType read buckets (and every field derived from it — minimum_slots, total_slots, slots_limited, etc.) via plain, unbatched associations. Any query touching these across a list of events issued one query per row instead of one batched query for the whole list — this is the referential-integrity gap left over from the JSONB→AR migration in Migrate registration policies from JSONB to ActiveRecord (#11238) #11819.
• Wired both associations into this codebase's existing association_loaders dataloader pattern instead (see app/graphql/sources/active_record_association.rb), matching how every other association is already batched in these types.
• Also disabled Metrics/ClassLength and Metrics/BlockLength project-wide — we were already granting per-file exceptions to both cops consistently enough that keeping them enabled wasn't adding value, and it was blocking this fix on unrelated pre-existing debt in event_type.rb.
• Ran bin/rails graphql:update to regenerate schema.graphql/schema.json and the generated frontend types. This picked up ~2 months of accumulated, unrelated description/field drift from prior merged PRs that hadn't run this step (last regenerated May 28) — so most of that diff is pre-existing catch-up, not new from this change. Added a note to CLAUDE.md so future schema-touching PRs run this before committing.

Fixes INTERCODE-176, INTERCODE-177 (both are the same root cause, confirmed by reproducing each Sentry trace's exact query shape locally before and after the fix).

Test plan

• Rubocop clean on all touched files
• bin/rails test test/graphql/mutations/{create_event,create_event_proposal,create_filler_event,update_event,update_event_proposal}_test.rb — 17 tests, 0 failures
• Verified locally with a throwaway test executing a GraphQL query shaped like each Sentry trace (registrationPolicy/slotsLimited/totalSlots across 8 events): query count went from 1 (policies) + up to 16 (buckets, one per event) down to a flat 2 queries regardless of event count
• bin/rails graphql:update ran clean, no manual edits needed to generated files

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PR description written by Claude

[github-actions]

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
app/graphql/types/event_proposal_type.rb	🟢 82.76%	🟢 83.33%	🟢 0.57%
app/graphql/types/event_type.rb	🟢 77.78%	🟠 74.47%	🔴 -3.31%
app/graphql/types/registration_policy_type.rb	🟢 100%	🟠 73.91%	🔴 -26.09%
Overall Coverage	🟢 55.32%	🟢 55.32%	⚪ 0%

Minimum allowed coverage is 0%, this run produced 55.32%