Add usage limits for traces (hard and soft caps)

[mmabrouk]

Context

Users on paid plans can currently exceed their monthly trace limit without any control over it. This came up from a customer who wanted to block traces once they hit their limit to avoid unexpected charges.

Requested behavior

Allow users to set a usage limit for traces at the project level. When the limit is reached, Agenta should either:

• Stop accepting new traces (hard cap), or
• Send a notification/alert and continue (soft cap)

Proposed solution

Add a usage limits setting in the project settings page. Users can configure:

• A hard limit to block traces beyond a threshold
• An optional alert threshold (e.g. notify at 80% usage)

The billing usage endpoint already returns the data needed to enforce this (value, limit, strict).

Why this matters

This gives customers cost predictability and control, which is important for teams running Agenta in production at scale.