docs: update implementation plan and add backend completion summary

- Mark backend as complete in IMPLEMENTATION_PLAN.md
- Create ADMIN_BACKEND_COMPLETE.md with full documentation
- Document all API endpoints
- Add testing instructions
- Add deployment notes

IMPLEMENTATION_PLAN.md

@@ -130,15 +130,15 @@ Tabungin is a personal finance SaaS with unique differentiators:
 - [ ] Phase 1: Admin Dashboard
   - [x] Schema migration ✅
   - [x] Seeder ✅
-  - [ ] Backend (IN PROGRESS)
+  - [x] Backend ✅
     - [x] Admin Guard
     - [x] JWT Role Support
-    - [ ] Plans CRUD
-    - [ ] Payment Methods CRUD
-    - [ ] Payments Verification
-    - [ ] Users Management
-    - [ ] App Config
-  - [ ] Frontend
+    - [x] Plans CRUD
+    - [x] Payment Methods CRUD
+    - [x] Payments Verification
+    - [x] Users Management
+    - [x] App Config
+  - [ ] Frontend (NEXT)
 
 **Next:**
 - [ ] Phase 2: Team