init: edu-content 内容库骨架（L1-L4 四层数据模型 + B类迁移 + lint/CI 工具）

sources/exams/cambridge-official.yaml

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+sources:
+  - id: cambridge-official
+    title: 剑桥官方 KET/PET 样卷、词表与听力音频
+    publisher: Cambridge Assessment English / UCLES
+    type: dataset
+    url: https://www.cambridgeenglish.org/Images/
+    license: official-free
+    license_note: "官方免费发布，允许教学使用（样卷/词表/听力音频）"
+    fetch_method: manual-download
+    status: fetched
+    fetched_at: "2026-08-21"
+    items:
+      - id: a2-key-vocab
+        title: A2 Key 2020 词汇表 PDF
+        oss_key: edu-raw/exams/ket/vocab/cambridge-official/a2-key-vocabulary-list.pdf
+        sha256: null
+        size: null
+        media_type: application/pdf
+        fetch_date: "2026-08-21"
+        status: planned
+        note: "vocab JSONL (structured/exams/ket/vocab/) 由此清洗；待 oss_sync 上传"
+      - id: b1-preliminary-vocab
+        title: B1 Preliminary 词汇表 PDF
+        oss_key: edu-raw/exams/pet/vocab/cambridge-official/b1-preliminary-vocabulary-list.pdf
+        sha256: null
+        size: null
+        media_type: application/pdf
+        fetch_date: "2026-08-21"
+        status: planned
+      - id: a2-key-listening
+        title: A2 Key 听力音频
+        oss_key: edu-raw/exams/ket/audio/cambridge-official/a2-key-listening.mp3
+        sha256: null
+        size: null
+        media_type: audio/mpeg
+        fetch_date: "2026-08-21"
+        status: planned
+      - id: sample-papers
+        title: 官方样卷包（PDF/MP3/ZIP）
+        oss_key: edu-raw/exams/ket/sample_papers/cambridge-official/
+        sha256: null
+        size: null
+        media_type: application/zip
+        fetch_date: "2026-08-21"
+        status: planned