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>

Cargo.toml

@@ -108,11 +108,6 @@ name = "question_lint"
 path = "src/bin/question_lint.rs"
 required-features = ["ssr"]
 
-[[bin]]
-name = "backfill_events"
-path = "src/bin/backfill_events.rs"
-required-features = ["ssr"]
-
 [profile.wasm-release]
 inherits = "release"
 opt-level = 'z'
@@ -121,9 +116,9 @@ codegen-units = 1
 
 [package.metadata.leptos]
 output-name = "portal"
-# Disambiguates the app binary from the two utility [[bin]] targets
-# (question_lint, backfill_events) added alongside it - cargo-leptos
-# otherwise can't tell which of the three is "the" server.
+# Disambiguates the app binary from the utility [[bin]] target
+# (question_lint) added alongside it - cargo-leptos otherwise can't
+# tell which of the two is "the" server.
 bin-target = "portal"
 site-root = "target/site"
 site-pkg-dir = "pkg"