feat: event infrastructure — payload, transport, NATS adapter

- EventPublisher now takes &DomainEvent (11 call sites + 3 impls updated)
- EventEnvelope + EventConsumer port in domain
- event-payload: serializable DomainEvent mirror with subject routing
- event-transport: generic Transport/MessageSource traits, publisher/consumer adapters
- adapters-nats: JetStream publish + durable pull consumer

Cargo.toml

@@ -7,6 +7,9 @@ members = [
     "crates/adapters/postgres",
     "crates/adapters/auth",
     "crates/adapters/storage",
+    "crates/adapters/event-payload",
+    "crates/adapters/event-transport",
+    "crates/adapters/nats",
     "crates/presentation",
     "crates/bootstrap",
     "crates/worker",
@@ -40,4 +43,9 @@ application   = { path = "crates/application" }
 api-types     = { path = "crates/api-types" }
 adapters-auth = { path = "crates/adapters/auth" }
 adapters-storage = { path = "crates/adapters/storage" }
+event-payload    = { path = "crates/adapters/event-payload" }
+event-transport  = { path = "crates/adapters/event-transport" }
+adapters-nats    = { path = "crates/adapters/nats" }
+async-nats       = "0.48"
+async-stream     = "0.3"
 presentation  = { path = "crates/presentation" }