feat: Prepare DB architecture for AI multimedia ecosystem

- Add ai_podcast_url and ai_video_url to lessons for NotebookLM integration
- Add markdown_content to lessons to store extracted PDF curriculum directly in DB
- Create lesson_resources table for managing Worksheets, Summaries, and Books PDFs

backend/database_schema.sql

@@ -218,11 +218,21 @@ CREATE TABLE IF NOT EXISTS `lessons` (
   `course_id` BIGINT UNSIGNED NOT NULL,
   `title` VARCHAR(255) NOT NULL,
   `sequence_order` INT UNSIGNED NOT NULL DEFAULT 1,
+  
+  -- المعلم الحقيقي (Teacher Video)
   `video_uuid` VARCHAR(64) DEFAULT NULL,
   `hls_url` VARCHAR(500) DEFAULT NULL,
   `timeline_chapters_json` JSON DEFAULT NULL,
   `bunny_video_id` VARCHAR(100) DEFAULT NULL,
   `duration_seconds` INT UNSIGNED NOT NULL DEFAULT 0,
+  
+  -- الذكاء الاصطناعي (NotebookLM / AI Audio-Video Explanations)
+  `ai_podcast_url` VARCHAR(500) DEFAULT NULL COMMENT 'رابط الشرح الصوتي المولد بالذكاء الاصطناعي (NotebookLM style)',
+  `ai_video_url` VARCHAR(500) DEFAULT NULL COMMENT 'رابط فيديو الشرح الآلي بالذكاء الاصطناعي',
+  
+  -- المحتوى النصي المفرغ من الـ PDF
+  `markdown_content` LONGTEXT DEFAULT NULL COMMENT 'محتوى الدرس النصي بصيغة Markdown و LaTeX',
+  
   `is_free_preview` TINYINT(1) NOT NULL DEFAULT 0,
   `created_at` TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP,
   `updated_at` TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
@@ -232,6 +242,22 @@ CREATE TABLE IF NOT EXISTS `lessons` (
   CONSTRAINT `fk_lessons_course` FOREIGN KEY (`course_id`) REFERENCES `courses` (`id`) ON DELETE CASCADE
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
 
+-- ------------------------------------------------------------------------------
+-- 10.5. Table: lesson_resources (المصادر المرفقة للدرس: كتب، ملخصات، أوراق عمل PDF)
+-- ------------------------------------------------------------------------------
+CREATE TABLE IF NOT EXISTS `lesson_resources` (
+  `id` BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
+  `lesson_id` BIGINT UNSIGNED NOT NULL,
+  `title` VARCHAR(255) NOT NULL,
+  `resource_type` ENUM('book_pdf', 'summary_pdf', 'worksheet_pdf', 'external_link') NOT NULL DEFAULT 'summary_pdf',
+  `file_url` VARCHAR(500) NOT NULL,
+  `is_ai_generated` TINYINT(1) NOT NULL DEFAULT 0,
+  `created_at` TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP,
+  PRIMARY KEY (`id`),
+  KEY `idx_resources_lesson` (`lesson_id`),
+  CONSTRAINT `fk_resources_lesson` FOREIGN KEY (`lesson_id`) REFERENCES `lessons` (`id`) ON DELETE CASCADE
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
+
 -- ------------------------------------------------------------------------------
 -- 11. Table: video_quality_assessments (تقييم جودة المحتوى التعليمي والتوافق بالذكاء الاصطناعي)
 -- ------------------------------------------------------------------------------