Make aggregate state graphs content-driven, not compiled Rust

Six hand-maintained copies of the bucket->aggregate-type table
(4 per-type State enums, content::aggregate_type_for_bucket +
is_valid_transition_target, answers.rs's two dispatch matches,
backfill_events.rs's own migration table) collapse into one:
questions/aggregates.yaml, loaded and hot-swapped in AppState.aggregates
the same way AppState.questions already is. aggregates/mod.rs's
replay/create/transition now run off a runtime-loaded AggregateSchema
instead of a compile-time AggregateKind trait impl per type - the
CAS/JetStream mechanics underneath are unchanged. A new aggregate type
is now a content-only change, no portal deploy required.

backfill_events.rs is deleted outright rather than ported - its
migration job was already done and production data here is minimal
and expendable (the EVENTS stream gets purged by hand post-deploy).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app.rs

@@ -1359,6 +1359,7 @@ pub async fn submit_answer(
     if let Some(bucket) = &record_as {
         if let Err(e) = store_answer(
             &state.jetstream,
+            &state.aggregates.load(),
             bucket,
             chain_hash.clone(),
             &question_id,