fix(rate-limit): tighten rate limits on test notification endpoints

[nora-shap]

The test-fire-action endpoints had no meaningful rate limiting to prevent abuse, let's tighten this up.

Before

Layer	Limit	Scope
Endpoint-level	Default (999/sec)	Per-user
creation_quota	1000/hr	Per-project

After

Layer	Limit	Scope
Endpoint-level	10/min	Per-user
Endpoint-level	50/min	Per-org
creation_quota	100/hr	Per-project

The new creation_quota aligns with other low-volume issue types (metric alerts, processing errors) that also use 100/hr.