feat: article attachments with content-addressed dedup

Add an attachments table and AJAX upload/management on the article
create and edit pages.

- Attachment model with article_id (0 while pending on create),
  session_token ownership, and SHA-256 content-addressed stored_name
- Upload validates via the platform policy (switch/type/size) and
  deduplicates on disk by content hash
- Plan-A binding: attachments uploaded before an article exists are
  owned by a session token and bound to the new article on save
- Delete soft-removes the record and drops the disk file only when no
  remaining rows reference it (reference counting for deduped files)
- Edit page loads existing attachments via JSON list endpoint
- Row actions: insert into body (markdown image/link), set as cover
  (image only), and delete
- Download URLs use the configured default download base URL when set,
  else the local /uploads path

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

models/db.go

@@ -45,7 +45,7 @@ func InitDB(cfg *config.Config) *gorm.DB {
 	}
 
 	// Auto-migrate tables (idempotent).
-	if err := db.AutoMigrate(&User{}, &Article{}, &SiteSetting{}, &UploadConfig{}, &UploadFileType{}, &DownloadBaseURL{}); err != nil {
+	if err := db.AutoMigrate(&User{}, &Article{}, &SiteSetting{}, &UploadConfig{}, &UploadFileType{}, &DownloadBaseURL{}, &Attachment{}); err != nil {
 		log.Fatalf("Failed to auto-migrate database: %v", err)
 	}