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/metadata_extractor.rs

@@ -0,0 +1,68 @@
+use async_trait::async_trait;
+use domain::{
+    entities::{AssetMetadata, MetadataSource},
+    errors::DomainError,
+    ports::{AssetMetadataRepository, AssetRepository, FileStoragePort, PluginExecutor},
+    value_objects::{MetadataValue, StructuredData, SystemId},
+};
+use std::sync::Arc;
+use tracing::info;
+
+pub struct MetadataExtractorPlugin {
+    asset_repo: Arc<dyn AssetRepository>,
+    file_storage: Arc<dyn FileStoragePort>,
+    metadata_repo: Arc<dyn AssetMetadataRepository>,
+}
+
+impl MetadataExtractorPlugin {
+    pub fn new(
+        asset_repo: Arc<dyn AssetRepository>,
+        file_storage: Arc<dyn FileStoragePort>,
+        metadata_repo: Arc<dyn AssetMetadataRepository>,
+    ) -> Self {
+        Self {
+            asset_repo,
+            file_storage,
+            metadata_repo,
+        }
+    }
+}
+
+#[async_trait]
+impl PluginExecutor for MetadataExtractorPlugin {
+    fn plugin_name(&self) -> &str {
+        "metadata_extractor"
+    }
+
+    async fn execute(
+        &self,
+        asset_id: Option<SystemId>,
+        _payload: &StructuredData,
+        _config: &StructuredData,
+    ) -> Result<StructuredData, DomainError> {
+        let asset_id = asset_id.ok_or_else(|| {
+            DomainError::Validation("metadata_extractor requires asset_id".into())
+        })?;
+
+        let asset = self
+            .asset_repo
+            .find_by_id(&asset_id)
+            .await?
+            .ok_or_else(|| DomainError::NotFound(format!("Asset {} not found", asset_id)))?;
+
+        let path = &asset.source_reference.relative_path;
+        let data = self.file_storage.read_file(path).await?;
+        let file_size = data.len() as i64;
+
+        let mut extracted = StructuredData::new();
+        extracted.insert("file_size_bytes", MetadataValue::Integer(file_size));
+        extracted.insert("mime_type", MetadataValue::String(asset.mime_type.clone()));
+
+        let metadata =
+            AssetMetadata::new(asset_id, MetadataSource::ExifExtracted, extracted.clone());
+        self.metadata_repo.save(&metadata).await?;
+
+        info!(asset_id = %asset_id, file_size, "extracted basic metadata");
+        Ok(extracted)
+    }
+}