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

crates/worker/src/plugins/no_op.rs

@@ -0,0 +1,26 @@
+use async_trait::async_trait;
+use domain::{
+    errors::DomainError,
+    ports::PluginExecutor,
+    value_objects::{StructuredData, SystemId},
+};
+use tracing::info;
+
+pub struct NoOpPlugin;
+
+#[async_trait]
+impl PluginExecutor for NoOpPlugin {
+    fn plugin_name(&self) -> &str {
+        "no_op"
+    }
+
+    async fn execute(
+        &self,
+        asset_id: Option<SystemId>,
+        _payload: &StructuredData,
+        _config: &StructuredData,
+    ) -> Result<StructuredData, DomainError> {
+        info!(asset_id = ?asset_id, "no_op plugin executed");
+        Ok(StructuredData::new())
+    }
+}