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

Cargo.toml

@@ -11,6 +11,7 @@ members = [
     "crates/adapters/event-transport",
     "crates/adapters/nats",
     "crates/adapters/exif",
+    "crates/adapters/thumbnail",
     "crates/presentation",
     "crates/bootstrap",
     "crates/worker",
@@ -47,7 +48,8 @@ adapters-storage = { path = "crates/adapters/storage" }
 event-payload    = { path = "crates/adapters/event-payload" }
 event-transport  = { path = "crates/adapters/event-transport" }
 adapters-nats    = { path = "crates/adapters/nats" }
-adapters-exif    = { path = "crates/adapters/exif" }
+adapters-exif      = { path = "crates/adapters/exif" }
+adapters-thumbnail = { path = "crates/adapters/thumbnail" }
 async-nats       = "0.48"
 async-stream     = "0.3"
 presentation  = { path = "crates/presentation" }