feat(streaming): generic stream responses (generators) for non-SSE chunks

[ZhuchkaTriplesix]

Resolves #103

• Adds oxyroute.streaming module with stream_bytes, stream_text, and stream_jsonl helpers.
• Uses Granian's response_stream to write chunks incrementally.
• Handlers returning streams must be async def and await the helpers to properly respect RSGI backpressure.
• Updated documentation (docs/streaming.md) to cover error and backpressure behavior.
• Added comprehensive unit tests in tests/test_streaming.py.