API: Implement Board paper management endpoints

[lucianfialho]

Problem

Users need to add and remove papers from their curated collections. Currently there are no endpoints for managing papers within a board.

Solution

Implement endpoints to manage the papers within a board.

Acceptance Criteria

• POST /api/boards/[id]/papers - Add paper to board

  • Request body: { paperId: string, notes?: string }
  • Response: { id, boardId, paperId, addedAt }
  • Checks if paper exists in papers table
  • Prevents duplicate paper entries (returns existing if already there)
  • Requires editor+ role on board

• DELETE /api/boards/[id]/papers/[paperId] - Remove paper

  • No request body needed
  • Returns 204 on success
  • Requires editor+ role on board
  • Returns 404 if paper not in board

• PATCH /api/boards/[id]/papers/[paperId] - Update paper metadata in board (optional)

  • Request body: { notes? }
  • Updates board-specific metadata about the paper
  • Requires editor+ role

• GET /api/boards/[id]/papers - List board papers

  • Query params: limit=20, offset=0, sort=added|title|authors
  • Response: { papers: PaperWithBoardMeta[], total: number }
  • Includes full paper details + board-specific data

Dependencies

• Issue Data Model: Design and implement Board entity and schema #1: Data Model must exist
• Issue API: Implement Board CRUD endpoints #2: Board CRUD endpoints

Notes

• When adding a paper, check if it exists in papers table; if not, fetch from arXiv/Crossref first
• Paper removal should cascade properly via db schema
• Maintain paper count on board for display purposes