Auto-capture + backfill TopicSubscriptions from the registration form's interested_in_more answer

Follow-up to #2070 (introduces TopicSubscription / TopicSubscriptionType).

Problem

The registration form's interested_in_more answer ("Are you interested in learning more about upcoming trainings or resources?") still drives nothing — it's stored only as an inert FormAnswer. And historical answers aren't backfilled into subscriptions. The target topic is already defined: TopicSubscriptionType::INTERESTED_IN_MORE_KEY = "facilitator_trainings".

Scope

1. Capture on submit — when a public/admin registration is submitted with interested_in_more: Yes, create (or reactivate) an active TopicSubscription for that person on the facilitator_trainings topic, with a traceable source (mirroring the record_mailing_list_consent provenance, e.g. "2026-08-03 Facilitator Training registration"). Idempotent — don't create a duplicate active subscription (the model already enforces one active per person·type·event).
2. Backfill migration — walk existing FormAnswers for interested_in_more == Yes and create the corresponding subscriptions, preserving the original registration date as subscribed_at.

Notes / open questions

• Resolve the canonical type via the stable key (TopicSubscriptionType.interested_in_more); guard for the type not existing (e.g. before seeds run).
• Confirm whether a No answer should do anything (likely nothing).
• Consent is separate: capturing interest ≠ mailing_list_consent. Don't conflate.
• Decide whether general (interested_event: nil) is correct here, or whether to narrow to the registration's event.

Blocked on #2070 merging (or can build on that branch).

Start by reviewing #2070 and the TopicSubscription and TopicSubscriptionType models, then trace public/admin registration submission and the existing record_mailing_list_consent provenance. Inspect FormAnswer records and the migration path before deciding how to handle missing types, No answers, and event scope. Done means idempotent capture and a backfill preserving the original registration date without conflating interest with consent.