Backpressure-aware request body streaming #358

Area: Http\Request, Http\Support\RequestParser, streaming responses

Problem

Doppar can stream files and JSON responses, but large uploads still encourage reading request content into memory before application code can process it.

Proposal

Expose an iterable request-body stream with configurable byte limits, idle timeouts, and cancellation. Add helpers for incremental JSON Lines and multipart processing while preserving the existing parsed-input API.

Acceptance criteria

• A 1 GB test fixture can be consumed without buffering the complete body.
• Limits are enforced before disk or application writes exceed the limit.
• Client disconnects close resources and stop downstream processing.
• Multipart boundaries and malformed input have explicit tests.