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/processing.rs

@@ -0,0 +1,22 @@
+use application::processing::{ExecutePipelineHandler, ProcessNextJobHandler};
+use domain::ports::{EventPublisher, PluginRegistry};
+use std::sync::Arc;
+
+use super::Repos;
+
+pub fn build_process_next_handler(
+    repos: &Repos,
+    registry: Arc<dyn PluginRegistry>,
+    event_pub: Arc<dyn EventPublisher>,
+) -> ProcessNextJobHandler {
+    let execute_pipeline = Arc::new(ExecutePipelineHandler::new(
+        repos.job.clone(),
+        repos.batch.clone(),
+        repos.pipeline.clone(),
+        repos.plugin.clone(),
+        registry,
+        event_pub,
+    ));
+
+    ProcessNextJobHandler::new(repos.job.clone(), execute_pipeline)
+}