backpressure, quotas, and overload protection

[ashupednekar]

Summary

Add backpressure and quota controls for ingress and runtime execution.

Motivation

Without limits, noisy tenants or malformed traffic can degrade global stability.

Proposal

• Per-project and per-function concurrency limits.
• Request payload size limits for sync/WS/SSE paths.
• Queue depth and consumer lag guardrails.
• Rate limiting at ingress layer.

Acceptance criteria

• Configurable limits are enforced with clear error responses.
• Overload protection prevents unbounded memory/queue growth.
• Limit-hit metrics are available for operators.