feat: thumbnail generator plugin with configurable size/format

- ThumbnailGeneratorPort in domain (bytes + config → resized bytes)
- adapters-thumbnail: ImageThumbnailGenerator using image crate
- ThumbnailGeneratorPlugin reads width/height/format/profile from step config
- PostgresDerivativeRepository + 012_derivatives migration
- Seeded in extract_metadata pipeline as step 2 (300x300 webp)
- Standalone generate_derivative pipeline for on-demand use

crates/worker/src/factories/plugins.rs

@@ -1,6 +1,8 @@
 use crate::plugin_registry::InMemoryPluginRegistry;
-use crate::plugins::{MetadataExtractorPlugin, NoOpPlugin, SidecarSyncPlugin};
-use domain::ports::{MetadataExtractorPort, SidecarWriterPort};
+use crate::plugins::{
+    MetadataExtractorPlugin, NoOpPlugin, SidecarSyncPlugin, ThumbnailGeneratorPlugin,
+};
+use domain::ports::{MetadataExtractorPort, SidecarWriterPort, ThumbnailGeneratorPort};
 use std::sync::Arc;
 
 use super::Repos;
@@ -10,16 +12,23 @@ pub fn build_plugin_registry(
     file_storage: Arc<dyn domain::ports::FileStoragePort>,
     sidecar_writer: Arc<dyn SidecarWriterPort>,
     extractor: Arc<dyn MetadataExtractorPort>,
+    thumbnail_gen: Arc<dyn ThumbnailGeneratorPort>,
 ) -> InMemoryPluginRegistry {
     let mut registry = InMemoryPluginRegistry::new();
 
     registry.register(Arc::new(NoOpPlugin));
     registry.register(Arc::new(MetadataExtractorPlugin::new(
         repos.asset.clone(),
-        file_storage,
+        file_storage.clone(),
         repos.metadata.clone(),
         extractor,
     )));
+    registry.register(Arc::new(ThumbnailGeneratorPlugin::new(
+        repos.asset.clone(),
+        file_storage,
+        repos.derivative.clone(),
+        thumbnail_gen,
+    )));
 
     let export_handler = Arc::new(application::sidecar::ExportSidecarHandler::new(
         repos.metadata.clone(),