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/domain/src/catalog/ports.rs

@@ -81,3 +81,22 @@ pub trait DuplicateRepository: Send + Sync {
 pub trait MetadataExtractorPort: Send + Sync {
     fn extract(&self, bytes: &Bytes) -> Result<StructuredData, DomainError>;
 }
+
+// --- ThumbnailGeneratorPort ---
+
+pub struct ThumbnailOutput {
+    pub bytes: Bytes,
+    pub width: u32,
+    pub height: u32,
+    pub mime_type: String,
+}
+
+pub trait ThumbnailGeneratorPort: Send + Sync {
+    fn generate(
+        &self,
+        source: &Bytes,
+        width: u32,
+        height: u32,
+        format: &str,
+    ) -> Result<ThumbnailOutput, DomainError>;
+}