chore: add plan files for Phase 3 advanced features

plans/phase-3-advanced-features/phase-2-sparklines.md

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+# Phase 2: Ping History Sparklines
+
+**Status:** ⬜ Not Started
+**Parent plan:** [PLAN.md](./PLAN.md)
+**Domain:** fullstack
+
+## Tasks
+- [ ] Task 1: Create `src/routes/api/apps/[id]/history/+server.ts` — GET last 24h of healthcheck results
+- [ ] Task 2: Create `src/lib/components/app/SparklineChart.svelte` — tiny inline SVG sparkline (green=up, red=down)
+- [ ] Task 3: Update `src/lib/components/widget/AppWidget.svelte` — show sparkline below status badge
+- [ ] Task 4: Update `src/lib/components/app/AppCard.svelte` — show sparkline on app cards
+- [ ] Task 5: Calculate and display uptime percentage (last 24h)
+- [ ] Task 6: Update healthcheck service to retain last 288 records per app (24h at 5min intervals)
+- [ ] Task 7: Add cleanup job to prune old AppStatus records beyond retention period
+- [ ] Task 8: Add i18n translations (EN/RU)
+
+## Handoff to Next Phase
+<!-- Filled in after completion -->