feat: Document video progress tracking APIs and update the main API endpoint list.

docs/api-docs/ERD_v2_improved.txt

@@ -153,6 +153,10 @@ Table lesson_progress {
   lesson_id int [not null, ref: > lessons.id]
   is_completed boolean [not null, default: false]
   completed_at datetime
+  video_progress_seconds int [default: 0, note: 'current playback position in seconds']
+  video_duration_seconds int [note: 'total video duration in seconds']
+  video_progress_percentage decimal(5,2) [note: 'calculated: (progress/duration)*100']
+  last_watched_at datetime [note: 'last time user watched this video']
   created_at datetime [not null, default: `now()`]
   updated_at datetime [not null, default: `now()`]
   
@@ -160,6 +164,7 @@ Table lesson_progress {
     (user_id, lesson_id) [unique]
     user_id
     lesson_id
+    last_watched_at
   }
 }
 

docs/api-docs/api_endpoints.md

@@ -70,6 +70,8 @@ Authorization: Bearer <token>
 | GET | `/students/courses/:courseId/learn` | 👨‍🎓 Student | Get course learning page (with lock status) |
 | GET | `/students/courses/:courseId/lessons/:lessonId` | 👨‍🎓 Student | Get lesson content (checks prerequisites) |
 | GET | `/students/courses/:courseId/lessons/:lessonId/access-check` | 👨‍🎓 Student | Check lesson access without loading content |
+| POST | `/students/lessons/:lessonId/progress` | 👨‍🎓 Student | Save video progress |
+| GET | `/students/lessons/:lessonId/progress` | 👨‍🎓 Student | Get video progress |
 | POST | `/students/courses/:courseId/lessons/:lessonId/complete` | 👨‍🎓 Student | Mark lesson as complete |
 
 ### Instructor Endpoints
@@ -303,13 +305,13 @@ Authorization: Bearer <token>
 
 ---
 
-## Total Endpoints: **93+**
+## Total Endpoints: **95+**
 
 - Authentication: 6
 - User Management: 4
 - Categories: 5
-- Courses: 12
-- Chapters & Lessons: 16 (-1)
+- Courses: 14 (+2)
+- Chapters & Lessons: 16
 - Quizzes: 10
 - Announcements: 6
 - Reports: 11

docs/api-docs/api_video_progress.md

@@ -0,0 +1,191 @@
+# Video Progress Tracking - API Usage Examples
+
+## Save Video Progress
+
+### POST `/students/lessons/:lessonId/progress`
+
+**Purpose**: บันทึกตำแหน่งการดูวีดีโอ (เรียกทุก 5 วินาที)
+
+#### Request:
+```http
+POST /api/students/lessons/5/progress
+Authorization: Bearer <token>
+Content-Type: application/json
+
+{
+  "progress_seconds": 450,
+  "duration_seconds": 900
+}
+```
+
+#### Response 200:
+```json
+{
+  "lesson_id": 5,
+  "video_progress_seconds": 450,
+  "video_duration_seconds": 900,
+  "video_progress_percentage": 50.00,
+  "is_completed": false,
+  "last_watched_at": "2024-12-24T14:00:00Z"
+}
+```
+
+#### Auto-Complete (90%+):
+```json
+{
+  "progress_seconds": 810,
+  "duration_seconds": 900
+}
+```
+
+**Response**:
+```json
+{
+  "lesson_id": 5,
+  "video_progress_seconds": 810,
+  "video_duration_seconds": 900,
+  "video_progress_percentage": 90.00,
+  "is_completed": true,
+  "completed_at": "2024-12-24T14:05:00Z"
+}
+```
+
+---
+
+## Get Video Progress
+
+### GET `/students/lessons/:lessonId/progress`
+
+**Purpose**: ดึงตำแหน่งการดูวีดีโอเพื่อเล่นต่อ
+
+#### Request:
+```http
+GET /api/students/lessons/5/progress
+Authorization: Bearer <token>
+```
+
+#### Response 200 (Has Progress):
+```json
+{
+  "lesson_id": 5,
+  "video_progress_seconds": 450,
+  "video_duration_seconds": 900,
+  "video_progress_percentage": 50.00,
+  "is_completed": false,
+  "last_watched_at": "2024-12-24T13:30:00Z"
+}
+```
+
+#### Response 200 (No Progress):
+```json
+{
+  "lesson_id": 5,
+  "video_progress_seconds": 0,
+  "video_duration_seconds": null,
+  "video_progress_percentage": 0,
+  "is_completed": false,
+  "last_watched_at": null
+}
+```
+
+---
+
+## Frontend Integration
+
+```javascript
+const VideoPlayer = ({ lessonId, videoUrl }) => {
+  const videoRef = useRef(null);
+  
+  // Load saved progress
+  useEffect(() => {
+    async function loadProgress() {
+      const res = await fetch(`/api/students/lessons/${lessonId}/progress`);
+      const data = await res.json();
+      
+      if (data.video_progress_seconds > 0) {
+        videoRef.current.currentTime = data.video_progress_seconds;
+      }
+    }
+    loadProgress();
+  }, [lessonId]);
+  
+  // Save progress every 5 seconds
+  useEffect(() => {
+    const interval = setInterval(() => {
+      if (videoRef.current && !videoRef.current.paused) {
+        saveProgress(
+          Math.floor(videoRef.current.currentTime),
+          Math.floor(videoRef.current.duration)
+        );
+      }
+    }, 5000);
+    
+    return () => clearInterval(interval);
+  }, []);
+  
+  async function saveProgress(currentTime, duration) {
+    await fetch(`/api/students/lessons/${lessonId}/progress`, {
+      method: 'POST',
+      headers: {
+        'Authorization': `Bearer ${token}`,
+        'Content-Type': 'application/json'
+      },
+      body: JSON.stringify({
+        progress_seconds: currentTime,
+        duration_seconds: duration
+      })
+    });
+  }
+  
+  return <video ref={videoRef} src={videoUrl} controls />;
+};
+```
+
+---
+
+## Modified Endpoints
+
+### GET `/students/courses/:courseId/learn`
+
+**Added**: `video_progress_percentage` และ `resume_from` ในแต่ละ lesson
+
+```json
+{
+  "lessons": [
+    {
+      "id": 1,
+      "title": "Lesson 1",
+      "type": "video",
+      "is_completed": true,
+      "video_progress_percentage": 100
+    },
+    {
+      "id": 2,
+      "title": "Lesson 2",
+      "type": "video",
+      "is_completed": false,
+      "video_progress_percentage": 50,
+      "resume_from": 450
+    }
+  ]
+}
+```
+
+### GET `/students/courses/:courseId/progress`
+
+**Added**: `recently_watched` section
+
+```json
+{
+  "course_id": 1,
+  "progress_percentage": 30,
+  "recently_watched": [
+    {
+      "lesson_id": 2,
+      "title": "Lesson 2",
+      "video_progress_percentage": 50,
+      "last_watched_at": "2024-12-24T14:00:00Z"
+    }
+  ]
+}
+```

docs/api-docs/edge_cases_quick_reference.md

@@ -16,6 +16,7 @@
 | 8 | [เข้าถึงข้อมูลข้ามหลักสูตร](#8-เข้าถึงข้อมูลข้ามหลักสูตร) | 🔴 สูง | Middleware ตรวจสอบ ownership |
 | 9 | [เพิ่มเนื้อหาหลังจบหลักสูตร](#9-เพิ่มเนื้อหาหลังจบหลักสูตร) | 🟡 ปานกลาง | เนื้อหาใหม่ = เสริม (ไม่บังคับ) |
 | 10 | [Maintenance Mode](#10-maintenance-mode) | 🟡 ปานกลาง | Auto-save + แจ้งเตือนล่วงหน้า |
+| 11 | [Video Progress Tracking](#11-video-progress-tracking) | 🟡 ปานกลาง | Save ทุก 5 วินาที + last_watched_at |
 
 ---
 
@@ -211,6 +212,34 @@ if (originalProgress === 100 && certificateIssued) {
 
 ---
 
+## 11. Video Progress Tracking
+
+**ปัญหา**: Student ดูวีดีโอค้างไว้หลายแท็บ หรือปิดเบราว์เซอร์กลางคัน
+
+**วิธีแก้**:
+```javascript
+// 1. Save progress ทุก 5 วินาที
+// 2. ใช้ last_watched_at เป็นตัวตัดสิน
+// 3. Auto-complete เมื่อดู 90%+
+// 4. Resume จากตำแหน่งล่าสุด
+```
+
+**Key Points**:
+- ✅ Save progress ทุก 5 วินาที (ไม่บ่อยเกินไป)
+- ✅ ใช้ `last_watched_at` ตัดสินว่า session ไหนล่าสุด
+- ✅ Auto-complete เมื่อดู ≥ 90%
+- ✅ Resume จากตำแหน่งที่บันทึกล่าสุด
+- ❌ ไม่ save เมื่อ video pause หรือ buffering
+
+**Concurrent Updates**:
+```javascript
+// แท็บ A: บันทึกที่ 5:00 (14:00:00)
+// แท็บ B: บันทึกที่ 3:00 (14:00:05)
+// → ใช้แท็บ B (last_watched_at ใหม่กว่า)
+```
+
+---
+
 ## 🔒 หลักการสำคัญ (Best Practices)
 
 ### 1. Soft Delete > Hard Delete