feat(worker): DLQ processor — exhausted events moved to failed_events with exponential retry

crates/worker/Cargo.toml

@@ -12,6 +12,7 @@ domain              = { workspace = true }
 application         = { workspace = true }
 nats                = { workspace = true }
 event-transport     = { workspace = true }
+event-payload       = { workspace = true }
 activitypub-base    = { workspace = true }
 activitypub         = { workspace = true }
 postgres            = { workspace = true }
@@ -22,6 +23,7 @@ futures             = { workspace = true }
 tracing             = { workspace = true }
 tracing-subscriber  = { workspace = true }
 dotenvy             = { workspace = true }
+serde_json          = { workspace = true }
 sqlx                = { workspace = true }
 
 [dev-dependencies]