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/main.rs

@@ -29,10 +29,12 @@ async fn main() -> anyhow::Result<()> {
         .unwrap_or_else(|_| "https://project.uhhm.no/uhhm/questions".to_string());
     let content_branch = std::env::var("CONTENT_BRANCH").unwrap_or_else(|_| "main".to_string());
     let gitea_base = content::gitea_api_base(&content_repo)?;
+    let aggregates = content::load_aggregates_from_gitea(&content_repo, &content_branch).await?;
     let questions = content::load_questions_from_gitea(&content_repo, &content_branch, "questions").await?;
-    content::validate_questions(&questions)?;
+    content::validate_questions(&questions, &aggregates)?;
     tracing::info!(count = questions.len(), repo = %content_repo, branch = %content_branch, "loaded content");
     let questions = Arc::new(arc_swap::ArcSwap::from_pointee(questions));
+    let aggregates = Arc::new(arc_swap::ArcSwap::from_pointee(aggregates));
 
     let nats_url =
         std::env::var("NATS_URL").unwrap_or_else(|_| "nats://127.0.0.1:4222".to_string());
@@ -66,6 +68,7 @@ async fn main() -> anyhow::Result<()> {
         content_branch,
         "questions".to_string(),
         questions.clone(),
+        aggregates.clone(),
     ));
 
     let state = AppState {
@@ -73,6 +76,7 @@ async fn main() -> anyhow::Result<()> {
         nats,
         jetstream,
         questions,
+        aggregates,
         gitea_base,
         oidc: oidc_state,
         garage,