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/Cargo.toml

@@ -16,6 +16,7 @@ adapters-storage   = { workspace = true }
 adapters-nats      = { workspace = true }
 event-transport    = { workspace = true }
 adapters-exif      = { workspace = true }
+adapters-thumbnail = { workspace = true }
 async-nats         = { workspace = true }
 
 futures           = { workspace = true }