[Feature] Similar paper retrieval via embeddings

[jerry609]

Source TODO

• 论文 Embedding 存储
• 相似度检索
• GET /api/research/papers/{paper_id}/similar
• 详情页 Related Papers

Scope

• Embedding model/table + async generation
• Similarity service and endpoint
• UI integration in paper detail page

Acceptance Criteria

• Related papers are semantically relevant and deterministic

[jerry609]

Implemented in commit 30833bd on branch feat/dailypaper-sse-stream: paper_embeddings table + migration, deterministic similarity service, /api/research/papers/{paper_id}/similar endpoint, and Related Papers integration on the detail page.