fix(server): map SparrowEngineError::NvjpegUnavailable to 503 (Phase E regression)

Phase E commit 8788425 added SparrowEngineError::NvjpegUnavailable(String) to
the shared sparrow-engine-types crate and emits it from sparrow-engine-gpu
{yolo,classifier,tiled}.rs JpegDecoder::new paths. sparrow-engine-server's
bongo_into_response() match in error.rs was missed in that change, so any
sparrow-engine-dev build of sparrow-engine-server (CPU or GPU) fails:

  error[E0004]: non-exhaustive patterns: `&sparrow_engine::SparrowEngineError::NvjpegUnavailable(_)` not covered
   --> sparrow-engine-server/src/error.rs:98:32

The match is shared across flavors because the variant exists unconditionally
in sparrow-engine-types (not behind cfg). CPU never emits it but still must
handle it for type exhaustiveness.

Mapping: 503 SERVICE_UNAVAILABLE + code 'NVJPEG_UNAVAILABLE' — analogous to
existing EngineFreed -> SERVICE_UNAVAILABLE 'ENGINE_UNAVAILABLE' (line 101).
Semantically the GPU resource (libnvjpeg.so.12) is required but unavailable;
operator can recover by installing the runtime via SPARROW_ENGINE_NVJPEG_LIBRARY_PATH.

Verification:
- cargo check -p sparrow-engine-server --no-default-features --features cpu: PASS
- ./scripts/test.sh -p sparrow-engine-server --lib --no-default-features --features cpu: 53/53 PASS

Root cause discovered while rebuilding sparrow-engine images from sparrow-engine-dev
HEAD for sparrow webapp pin sync (sparrow side: chore/sparrow-engine-sync-2026-05-27).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: zhmiao

sparrow-engine/sparrow-engine-server/src/error.rs

@@ -134,6 +134,8 @@ fn bongo_into_response(e: SparrowEngineError) -> Response {
         ImageDecode(_) => (StatusCode::UNPROCESSABLE_ENTITY, "IMAGE_DECODE_ERROR"),
         InvalidStride { .. } => (StatusCode::UNPROCESSABLE_ENTITY, "INVALID_IMAGE_INPUT"),
         ImageFileNotFound(_) => (StatusCode::NOT_FOUND, "IMAGE_NOT_FOUND"),
+        // GPU resources
+        NvjpegUnavailable(_) => (StatusCode::SERVICE_UNAVAILABLE, "NVJPEG_UNAVAILABLE"),
         // ORT / IO
         Ort(_) => (StatusCode::INTERNAL_SERVER_ERROR, "INFERENCE_ERROR"),
         Io(_) => (StatusCode::INTERNAL_SERVER_ERROR, "IO_ERROR"),