C-152: Post-launch monitoring window + on-call

**Owner:** @kh0ra  ·  **Reviewer:** @mehmethayirli
**Milestone:** M11 Launch

72h heightened monitoring; on-call rota; incident template ready.

**Acceptance criteria**
- On-call + monitoring active for 72h.

_Tracked in docs/MAINNET_ROADMAP.md (C-152)._