Update docs and guidelines for statistics feature

- CODE_GUIDELINES.md and feature-roadmap.md
- Add statistics-page-implementation-plan.md

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/feature-roadmap.md

@@ -291,10 +291,10 @@
 #### 10. **Reporting & Analytics** 📊
 
 **Current State:**
-- ❌ No reporting features
+- ✅ **Statistics page (MVP)** – `/statistics` with active/inactive member counts, joins/exits by year, cycle totals, open amount (2026-02-10)
 
 **Missing Features:**
-- ❌ Member statistics dashboard
+- ❌ Extended member statistics dashboard
 - ❌ Membership growth charts
 - ❌ Payment reports
 - ❌ Custom report builder