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>

i18n/i18n.go

@@ -121,6 +121,17 @@ var translations = map[Lang]map[string]string{
 		"article_content_required": "Content is required.",
 		"article_back_home":      "Back to Home",
 		"article_not_found":      "Article not found.",
+		"article_attachments":    "Attachments",
+		"article_upload":         "Upload",
+		"article_att_name":       "File",
+		"article_att_size":       "Size",
+		"article_att_insert":     "Insert",
+		"article_att_set_cover":  "Set as Cover",
+		"article_att_cover_set":  "Cover URL set.",
+		"article_att_pick":       "Please choose a file.",
+		"article_att_uploading":  "Uploading...",
+		"article_att_error":      "Upload failed. Please try again.",
+		"article_att_delete_confirm": "Delete this attachment?",
 
 		// Article management
 		"article_list_title":      "Articles",
@@ -293,6 +304,17 @@ var translations = map[Lang]map[string]string{
 		"article_content_required": "正文不能为空。",
 		"article_back_home":      "返回首页",
 		"article_not_found":      "文章不存在。",
+		"article_attachments":    "附件",
+		"article_upload":         "上传",
+		"article_att_name":       "文件",
+		"article_att_size":       "大小",
+		"article_att_insert":     "插入正文",
+		"article_att_set_cover":  "设为封面",
+		"article_att_cover_set":  "已设为封面。",
+		"article_att_pick":       "请先选择文件。",
+		"article_att_uploading":  "上传中...",
+		"article_att_error":      "上传失败，请重试。",
+		"article_att_delete_confirm": "删除该附件？",
 
 		// 文章管理
 		"article_list_title":      "文章管理",