Fix PWA energy kills with visibility-aware background pausing

[selfcontained]

Summary

• Close SSE EventSource when page is hidden, reopen on visible — prevents Safari from seeing sustained network activity in background PWAs
• Clear health poll interval when hidden (instead of just skipping inside the callback) — eliminates 8s timer fires entirely while backgrounded
• Guard WebSocket reconnect timers with document.hidden check — prevents reconnect loops from firing in the background
• Add visibility check to release manager 2s polling — stops aggressive polling when tab is hidden
• Pause CSS animations when hidden via html[data-hidden] attribute — stops GPU-composited will-change: transform animation from consuming energy
• Add energy metrics tracker (web/src/lib/energy-metrics.ts) that records SSE events, reconnects, HTTP requests, health poll fires/skips, visibility transitions, and hidden duration. Persists to localStorage every 30s and beacons to /api/v1/energy-report on visibility change to hidden — survives Safari's forced reload for post-mortem diagnostics
• Add backend /api/v1/energy-report endpoint that logs beacon payloads for server-side inspection
• Add mandatory pre-completion checks to CLAUDE.md and AGENTS.md — agents must now run type checking, web finalization, e2e tests, and unit tests before marking any task as done

Context

Safari's energy watchdog kills PWAs that consume "significant energy" while backgrounded. The app had several sources of continuous background activity: an always-open SSE connection, 8s health poll timer, WebSocket reconnect loops, and infinite CSS animations with will-change: transform.

Test plan

• npm run finalize:web passes (TypeScript check + production build)
• All 5 new e2e tests pass (e2e/energy-visibility.spec.ts)
• All 22 e2e tests pass (1 pre-existing flaky sidebar width test was also passing at time of final run)
• Verified data-hidden attribute toggles correctly via Playwright
• Verified energy metrics persist to localStorage with correct counters
• Manual PWA test on iOS Safari to confirm the energy kill no longer occurs

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