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/postgres/migrations/012_derivatives.sql

@@ -0,0 +1,14 @@
+CREATE TABLE derivatives (
+    derivative_id     UUID PRIMARY KEY,
+    parent_asset_id   UUID NOT NULL REFERENCES assets(asset_id),
+    profile_type      TEXT NOT NULL,
+    storage_path      TEXT NOT NULL,
+    mime_type         TEXT NOT NULL DEFAULT '',
+    file_size         BIGINT NOT NULL DEFAULT 0,
+    width             INTEGER NOT NULL DEFAULT 0,
+    height            INTEGER NOT NULL DEFAULT 0,
+    generation_status TEXT NOT NULL DEFAULT 'pending'
+);
+
+CREATE INDEX idx_derivatives_parent ON derivatives(parent_asset_id);
+CREATE INDEX idx_derivatives_parent_profile ON derivatives(parent_asset_id, profile_type);