docs: Add Forgejo and development setup guides, and refactor API documentation structure.

docs/ERD_v2_improved.txt

@@ -66,6 +66,9 @@ Table lessons {
   content jsonb [note: 'multi-language lesson content']
   type varchar [not null, note: 'video | pdf | text | quiz']
   sort_order int [not null, default: 0, note: 'must be >= 0']
+  is_sequential boolean [not null, default: true, note: 'require previous lessons to be completed']
+  prerequisite_lesson_ids jsonb [note: 'array of lesson IDs that must be completed first, e.g. [1, 2, 3]']
+  require_pass_quiz boolean [default: false, note: 'require passing quiz to unlock next lessons']
   created_at datetime [not null, default: `now()`]
   updated_at datetime [not null, default: `now()`]
   
@@ -214,6 +217,23 @@ Table announcement_attachments {
   }
 }
 
+Table lesson_attachments {
+  id int [pk, increment]
+  lesson_id int [not null, ref: > lessons.id]
+  file_name varchar [not null]
+  file_path varchar [not null, note: 'S3 key or file path']
+  file_size int [not null, note: 'in bytes']
+  mime_type varchar [not null]
+  description jsonb [note: 'multi-language description']
+  sort_order int [not null, default: 0]
+  created_at datetime [not null, default: `now()`]
+  
+  indexes {
+    lesson_id
+    (lesson_id, sort_order)
+  }
+}
+
 Table orders {
   id int [pk, increment]
   user_id int [not null, ref: > users.id]