Usage metering + open-core boundary

[AKogut]

Context

Foundation for SaaS monetization without crippling OSS. Meter operational cost drivers (ingestion volume, AI spend, retention).

Scope

• Metering events (executions ingested, artifacts stored, LLM tokens)
• Plan/quota model + soft/hard limits
• Clear code boundary: OSS core vs commercial modules (feature flags / separate package)
• Billing provider hooks (Stripe) behind an interface

Acceptance criteria

• Usage is measurable per tenant; OSS remains fully functional for self-host