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/factories/plugins.rs

@@ -0,0 +1,30 @@
+use crate::plugin_registry::InMemoryPluginRegistry;
+use crate::plugins::{MetadataExtractorPlugin, NoOpPlugin, SidecarSyncPlugin};
+use domain::ports::SidecarWriterPort;
+use std::sync::Arc;
+
+use super::Repos;
+
+pub fn build_plugin_registry(
+    repos: &Repos,
+    file_storage: Arc<dyn domain::ports::FileStoragePort>,
+    sidecar_writer: Arc<dyn SidecarWriterPort>,
+) -> InMemoryPluginRegistry {
+    let mut registry = InMemoryPluginRegistry::new();
+
+    registry.register(Arc::new(NoOpPlugin));
+    registry.register(Arc::new(MetadataExtractorPlugin::new(
+        repos.asset.clone(),
+        file_storage,
+        repos.metadata.clone(),
+    )));
+
+    let export_handler = Arc::new(application::sidecar::ExportSidecarHandler::new(
+        repos.metadata.clone(),
+        repos.sidecar.clone(),
+        sidecar_writer,
+    ));
+    registry.register(Arc::new(SidecarSyncPlugin::new(export_handler)));
+
+    registry
+}