docs: convert all ASCII charts to Mermaid diagrams

Converted 40+ ASCII art diagrams across 12 files to Mermaid:
- flowchart TD/LR for process flows and architecture
- erDiagram for entity relationships
- graph TD for tree structures (binary tree, categories)
- gantt for roadmap sprints

Files: BUSINESS-01 to 05, TECH-01/03/04/05, OVERVIEW-01/02/05
Directory tree structures kept as plain code blocks (Mermaid N/A)

overview/OVERVIEW-05-ROADMAP.md

@@ -69,40 +69,42 @@ Documentation     ████████████████████
 
 ### Q1 1404 (فروردین-خرداد)
 
-```
-Sprint 1 (فروردین):
-    ├── H2: Mobile Responsive
-    ├── H1: Product Bundle
-    └── M4: SEO Meta Tags
-
-Sprint 2 (اردیبهشت):
-    ├── M1: Manual Payment (if approved)
-    ├── M2: SignalR Chatika
-    └── M6: Dashboard Charts
-
-Sprint 3 (خرداد):
-    ├── M3: Dark Mode
-    ├── H3: Rate Limiting
-    └── L1: PWA
+```mermaid
+gantt
+    title Q1 1404 — فروردین تا خرداد
+    dateFormat YYYY-MM-DD
+    section Sprint 1 فروردین
+        H2 Mobile Responsive     :a1, 2025-03-21, 5d
+        H1 Product Bundle        :a2, after a1, 3d
+        M4 SEO Meta Tags         :a3, after a2, 2d
+    section Sprint 2 اردیبهشت
+        M1 Manual Payment        :b1, 2025-04-21, 3d
+        M2 SignalR Chatika       :b2, after b1, 2d
+        M6 Dashboard Charts      :b3, after b2, 3d
+    section Sprint 3 خرداد
+        M3 Dark Mode             :c1, 2025-05-22, 2d
+        H3 Rate Limiting         :c2, after c1, 2d
+        L1 PWA                   :c3, after c2, 3d
 ```
 
 ### Q2 1404 (تیر-شهریور)
 
-```
-Sprint 4 (تیر):
-    ├── L3: Monitoring
-    ├── L4: Log Aggregation
-    └── L2: API Versioning
-
-Sprint 5 (مرداد):
-    ├── L5: Product Compare
-    ├── L6: Wishlist
-    └── L7: Email Templates
-
-Sprint 6 (شهریور):
-    ├── L8: Refund System
-    ├── Performance Optimization
-    └── Security Audit
+```mermaid
+gantt
+    title Q2 1404 — تیر تا شهریور
+    dateFormat YYYY-MM-DD
+    section Sprint 4 تیر
+        L3 Monitoring            :d1, 2025-06-22, 3d
+        L4 Log Aggregation       :d2, after d1, 3d
+        L2 API Versioning        :d3, after d2, 2d
+    section Sprint 5 مرداد
+        L5 Product Compare       :e1, 2025-07-23, 2d
+        L6 Wishlist              :e2, after e1, 2d
+        L7 Email Templates       :e3, after e2, 2d
+    section Sprint 6 شهریور
+        L8 Refund System         :f1, 2025-08-23, 5d
+        Performance Optimization :f2, after f1, 3d
+        Security Audit           :f3, after f2, 3d
 ```
 
 ---