docs: record deferred improvements with resume checklists

[bolin8017]

Summary

Captures three review items that were investigated and intentionally deferred:

• Next.js 16 Cache Components: a full migration was attempted on branch `feat/next16-cache-components` (setting `cacheComponents: true` + 'use cache' helpers across public pages + tag invalidation in `deleteProblem`). The attempt was reverted because `cacheComponents: true` requires every uncached data access to sit inside a `` boundary during static generation — that's a 20+ file UI refactor across admin and mixed public/auth pages rather than a drop-in migration. Doc captures what was tried and a resume checklist.
• Branded channel-identifier types: narrow compile-time win, wide refactor surface.
• Node 22 → 24 + pnpm 9 → 10: needs real install + Playwright regression; checklist captured.

Test plan

• Docs-only change; no code impact
• Manually inspected `docs/deferred-improvements.md`

🤖 Generated with Claude Code

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Project	Deployment	Actions	Updated (UTC)
caffecode	Ready	Preview, Comment	Apr 18, 2026 3:34am

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📊 Coverage Report

Package	Statements	Branches	Functions	Lines
shared	92.22% ❌ (≥95%)	87.74% ❌ (≥90%)	96.87% ✅ (≥95%)	91.88% ❌ (≥95%)
web	95.47% ✅ (≥90%)	90.75% ✅ (≥85%)	96.42% ✅ (≥90%)	96.18% ✅ (≥90%)

_{🤖 Generated by CI · thresholds enforced via vitest}