feat: 书籍封面与音频附件、播放器与播放位置 (#21)

- schema v8：lexgo_book_attachments（按 book_id+kind 唯一，bytes MEDIUMBLOB）与
  lexgo_playback_positions（owner_id+book_id 主键），只用可重放 DDL，外键级联删书
- 后端：按文件头 magic bytes 判定类型（MP3 / JPG / PNG / WebP，含 WebP 维度解析），
  音频 ≤20 MiB、封面 ≤2 MiB 且 ≤4096²；上传/替换/移除；binaryResponse 交给
  http.ServeContent 提供 Range 206、416、ETag 与 304；播放位置 upsert，替换音频时重置
- 学习端：带凭据的 fetch + 对象 URL（令牌不进 URL），书库封面、书籍页「音频与封面」
  区块、阅读页常驻播放器（播放/暂停、进度、0.75–1.5 倍速、错误重试、位置上报）
- 测试：Go 86 项（新增附件单测/集成与 v7→v8 迁移）、学习端 157 单测与 26 项 E2E
- 顺带修掉 e2e/phrase.spec.ts 的偶发失败：重试条件改为断言期望词数、重读坐标、关面板后重试
- 文档：Architecture / Business-Rules / Local-Development / Requirements / Home /
  Deployment-and-Operations（备份范围含附件与体积提示）

scripts/ops.py

@@ -42,6 +42,7 @@ TABLES = [
     "sys_user", "lexgo_spaces", "lexgo_sessions", "lexgo_login_logs", "lexgo_operation_logs",
     "lexgo_books", "lexgo_chapters", "lexgo_ingest_jobs", "lexgo_dictionaries",
     "lexgo_terms", "lexgo_term_reviews", "lexgo_review_answers", "lexgo_chapter_progress",
+    "lexgo_book_attachments", "lexgo_playback_positions",
 ]
 SCHEMA_VERSION = 7
 
@@ -470,6 +471,9 @@ def integrity_checks(database):
     add("等级只出现在学习中词条", int(scalar(database, "SELECT COUNT(*) FROM lexgo_terms WHERE (status='learning' AND (level<1 OR level>7)) OR (status<>'learning' AND level<>0)", "0") or 0) == 0)
     add("完成记录都指向存在的章节", int(scalar(database, "SELECT COUNT(*) FROM lexgo_chapter_progress p LEFT JOIN lexgo_chapters c ON c.id=p.chapter_id WHERE c.id IS NULL", "0") or 0) == 0)
     add("词条语言与所属空间一致", int(scalar(database, "SELECT COUNT(*) FROM lexgo_terms t JOIN lexgo_spaces s ON s.owner_id=t.owner_id WHERE t.language<>s.language", "0") or 0) == 0)
+    add("附件都指向存在的书", int(scalar(database, "SELECT COUNT(*) FROM lexgo_book_attachments a LEFT JOIN lexgo_books b ON b.id=a.book_id WHERE b.id IS NULL", "0") or 0) == 0)
+    add("附件归属与书归属一致", int(scalar(database, "SELECT COUNT(*) FROM lexgo_book_attachments a JOIN lexgo_books b ON b.id=a.book_id WHERE a.owner_id<>b.owner_id", "0") or 0) == 0)
+    add("播放位置都指向存在的书", int(scalar(database, "SELECT COUNT(*) FROM lexgo_playback_positions p LEFT JOIN lexgo_books b ON b.id=p.book_id WHERE b.id IS NULL", "0") or 0) == 0)
 
     # The audit tables must not gain a column that could hold a credential or private content.
     for table in ("lexgo_login_logs", "lexgo_operation_logs"):