Allow freezing buckets in multi-run events

[nbudin]

Bug report

A user reported that events with multiple runs cannot freeze buckets. This is a known limitation: freezing bucket state is currently tracked on the event's registration policy, which is shared across all runs. Since freezing affects which signups can be moved between buckets, it must be applied at the run level.

Proposed solution

Once individual runs can override the event's registration policy (#11764), bucket freezing for multi-run events becomes feasible:

1. Each run tracks its own frozen/unfrozen state independently.
2. When a run is frozen, "no preference" signups in flex buckets are assigned to specific buckets, and the flex slots are converted into fixed slots in the target buckets — same logic as freezing on single-run events.

Depends on

• Migrate registration policies from JSONB to database tables #11238 (Migrate registration policies from JSONB to database tables)
• Allow individual runs to override the event's registration policy #11764 (Allow individual runs to override the event's registration policy)