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/adapters/thumbnail/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "adapters-thumbnail"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+domain = { workspace = true }
+bytes  = { workspace = true }
+image  = "0.25"