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/domain/src/processing/ports.rs

@@ -1,7 +1,8 @@
 use super::entities::{Job, JobBatch, Plugin, ProcessingPipeline};
 use crate::common::errors::DomainError;
-use crate::common::value_objects::SystemId;
+use crate::common::value_objects::{StructuredData, SystemId};
 use async_trait::async_trait;
+use std::sync::Arc;
 
 // --- JobRepository ---
 
@@ -38,3 +39,23 @@ pub trait PipelineRepository: Send + Sync {
     async fn find_by_trigger(&self, event: &str) -> Result<Vec<ProcessingPipeline>, DomainError>;
     async fn save(&self, pipeline: &ProcessingPipeline) -> Result<(), DomainError>;
 }
+
+// --- PluginExecutor ---
+
+#[async_trait]
+pub trait PluginExecutor: Send + Sync {
+    fn plugin_name(&self) -> &str;
+    async fn execute(
+        &self,
+        asset_id: Option<SystemId>,
+        payload: &StructuredData,
+        config: &StructuredData,
+    ) -> Result<StructuredData, DomainError>;
+}
+
+// --- PluginRegistry ---
+
+pub trait PluginRegistry: Send + Sync {
+    fn get_executor(&self, plugin_name: &str) -> Option<Arc<dyn PluginExecutor>>;
+    fn registered_plugins(&self) -> Vec<String>;
+}