docs: update DEVELOPMENT_PLAN.md and README.md for Issue #20 completion

Mark Issue #20 (History View & CSV Export) as complete in development plan.
Update README.md to reflect new frontend capabilities including:
- History page with interactive date range picker
- CSV export functionality
- Updated API endpoint documentation

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: hemanandr

DEVELOPMENT_PLAN.md

@@ -136,7 +136,7 @@ You can effectively work on **up to 6 parallel worktrees** without conflicts:
 | #17 | P1 | 4-6h | App shell, routing, layout | 5 | ✅ **COMPLETE** - Closed |
 | #18 | P1 | 1d | Live board dashboard page | 5 | ✅ **COMPLETE** - Live data integration working |
 | #19 | P2 | 1d | Endpoint detail page | 5 | ✅ **COMPLETE** - Full functionality with fallback |
-| #20 | P1 | 1d | History view & CSV export | 5 | 🔓 **UNLOCKED** |
+| #20 | P1 | 1d | History view & CSV export | 5 | ✅ **COMPLETE** |
 
 **Phase 6 Summary**: Core frontend infrastructure is complete and operational:
 - React 19 + TypeScript + Chakra UI v3 setup with Vite
@@ -145,13 +145,21 @@ You can effectively work on **up to 6 parallel worktrees** without conflicts:
 - App shell with navigation, routing, and responsive layout
 - Live status table and card views with sparkline charts
 - Environment-based configuration with .env support
-- **NEW**: Endpoint detail page with comprehensive monitoring data (Issue #19):
+- **Endpoint detail page** with comprehensive monitoring data (Issue #19):
   - Dynamic routing for `/endpoints/{id}` with React Router v7
   - Real-time refresh with 10-second polling using React Query
   - Recent checks timeline, outage tracking, and performance statistics
   - Intelligent fallback mechanism for missing backend endpoints
   - Responsive design optimized for desktop, tablet, and mobile
   - Full end-to-end testing with Puppeteer verification
+- **NEW: History View & CSV Export** with complete functionality (Issue #20):
+  - Interactive date range picker with quick presets (1h, 24h, week, month)
+  - Bucket selector for data granularity (raw, 15-minute, daily)
+  - Custom SVG availability charts with area visualization
+  - Comprehensive history table with pagination and sorting
+  - Client-side CSV export with full data formatting
+  - Performance statistics dashboard showing uptime trends
+  - Responsive design for all screen sizes with proper error handling
 
 ### PHASE 7: Service & Deployment (Week 3, Days 3-5)
 **Windows service - EPIC #8**

README.md

@@ -55,9 +55,15 @@ npm run dev
 
 ## API Endpoints
 
-The server provides REST API endpoints for configuration management:
+The server provides REST API endpoints for configuration management and monitoring data:
 
 ```bash
+# Live monitoring data
+GET /api/status/live
+
+# Historical data by endpoint  
+GET /api/history/endpoint/{id}
+
 # Apply YAML configuration
 POST /api/config/apply
 Content-Type: text/plain
@@ -115,7 +121,7 @@ GET /api/test/monitoring/outages
 - **Network Monitoring**: ✅ ICMP ping, TCP connect, HTTP status checks with concurrent execution
 - **Configuration**: ✅ YAML-based with JSON Schema validation and version tracking
 - **Data Storage**: ✅ SQLite with automatic 15-minute/daily rollups running every 5 minutes
-- **Web Interface**: ✅ Real-time status dashboard with live data integration, endpoint detail pages, and responsive layout
+- **Web Interface**: ✅ Real-time status dashboard with live data integration, endpoint detail pages, history view with CSV export, and responsive layout
 - **Configuration Management**: ✅ Apply, list, and download configuration versions
 - **Settings Management**: ✅ Key-value store with watermark tracking for rollup jobs
 - **Alerting**: ✅ Status change detection with flap damping (2/2 thresholds)