Fix review issues: async safety, schemas, model relations, and storage

[Iventyk]

Motivation

• Address several review findings: missing bidirectional relationship between Document and ChunkEmbedding, duplicated schema types/fields, and fragile filename parsing.
• Eliminate synchronous blocking calls and unsafe global state that can block the event loop or introduce race conditions in Celery workers.
• Make file storage scalable for large uploads and ensure API serialization/validation behaves predictably for aliased fields.

Description

• Added a bidirectional SQLAlchemy relationship between Document and ChunkEmbedding and removed the duplicated source_document column usage so chunk rows use chunk.document.name instead of a separate string field.
• Consolidated duplicate error response schemas into a single ErrorResponse and re-exported it for ask schema compatibility.
• Enabled Pydantic model population by name for DocumentListItem with populate_by_name=True so the chunks alias maps to chunks_count reliably during (de)serialization.
• Replaced per-endpoint manual service construction with FastAPI Depends provider helpers so routers use Depends(get_document_service) / Depends(get_question_answer_service) instead of calling Service(db) directly.
• Hardened document upload by parsing filename suffix with Path(...).suffix, adding enqueue error handling around process_document_task.delay(...), and marking documents failed with error_message on enqueue errors.
• Removed event-loop blocking synchronous calls by offloading embedding/LLM/loader/ splitter calls to asyncio.to_thread(...) and implemented an async _agenerate in the demo EchoChatModel.
• Rewrote FileStorageService.save to stream writes in 1MB chunks via thread offload to avoid reading entire uploads into memory, and made remove() asynchronous.
• Protected the Celery worker event loop global (_worker_loop) with a threading.Lock to avoid race conditions during initialization and shutdown.

Testing

• Ran formatter: black on modified files (reformatted src/services/storage.py) and completed without blocking.
• Verified bytecode compilation with python -m compileall src, which completed successfully.
• Linted the code with flake8 src, which returned clean results after fixes.

---

Codex Task