feat: add OpenAPI spec generation with utoipa and justfile

- Add utoipa v5 dependency with ToSchema derives on all REST API DTOs
- Annotate free-function handlers with #[utoipa::path] (trash, share, favorites, recent)
- Create ApiDoc struct with OpenApi derive registering 37 schemas across 7 tags
- Add generate-openapi binary outputting resources/gen/openapi.json
- Serve OpenAPI spec at GET /api/openapi.json (public, no auth)
- Add justfile with common dev commands (build, test, lint, check, openapi, db)

justfile

@@ -0,0 +1,47 @@
+set dotenv-load
+
+default:
+    @just --list
+
+build:
+    cargo build
+
+release:
+    cargo build --release
+
+run:
+    cargo run
+
+run-debug:
+    RUST_LOG=debug cargo run
+
+test:
+    cargo test --workspace
+
+test-mocks:
+    cargo test --features test_utils
+
+test-one name:
+    cargo test {{name}}
+
+fmt:
+    cargo fmt --all
+
+fmt-check:
+    cargo fmt --all --check
+
+lint:
+    cargo clippy -- -D warnings
+
+check:
+    cargo fmt --all
+    cargo clippy -- -D warnings
+
+openapi:
+    cargo run --bin generate-openapi
+
+db:
+    docker compose up -d postgres
+
+db-down:
+    docker compose down