feat: worker plugin system — domain ports, pipeline executor, built-in plugins

- PluginExecutor + PluginRegistry ports in domain
- ExecutePipelineCommand orchestrates job→pipeline→plugin steps
- ProcessNextJobCommand polls + executes next queued job
- InMemoryPluginRegistry, NoOp/MetadataExtractor/SidecarSync plugins
- Worker main rewritten with poll loop, factories module for DI
- Deleted template job/runner/jobs remnants

Cargo.lock

@@ -3061,7 +3061,9 @@ name = "worker"
 version = "0.1.0"
 dependencies = [
  "adapters-postgres",
+ "adapters-storage",
  "anyhow",
+ "application",
  "async-trait",
  "domain",
  "dotenvy",