Add end-to-end and go-live review reports (2026-04-26)

[XingCheng3]

Motivation

• Capture a deep end-to-end review of the current codebase with prioritized findings and concrete remediation steps in a persistent document.
• Record a final go/no-go assessment for production rollout including blocking issues and a minimum checklist for safe launch.
• Provide an actionable roadmap to address high‑risk areas such as concurrency, scheduling idempotency, data semantics, and backup/cleanup safety.

Description

• Add END_TO_END_REVIEW_2026-04-26.md containing a detailed end-to-end audit with identified P0/P1/P2 issues and specific fix recommendations for concurrency, manual triggers, duplicate task execution, config key mismatches, pseudo-foreign-key semantics, backup WITH FORMAT, cleanup is_pinned, time handling, and UX gaps.
• Add GO_LIVE_REVIEW_2026-04-26.md containing a go-live assessment that concludes No-Go with five blocking items (B1–B5), an expanded checklist for minimum launch criteria, and recommended conditional rollout steps.
• Both documents include prioritized remediation plans and a suggested multi-phase roadmap to stabilize the system before production traffic.

Testing

• This is a documentation-only change, so no automated tests were modified or required for this PR.
• No automated test runs were executed as part of this change.

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Codex Task