Implement Git Bridge API v0 POST endpoint for bidirectional sync

[Copilot]

PR #2 implemented GET endpoints for git-bridge to pull from Overleaf. This completes the implementation by adding the POST endpoint for pushing git changes back to Overleaf projects.

Changes

• POST /api/v0/docs/:project_id/snapshots - Accepts file changes from git-bridge

  • Version validation: Returns 409 if latestVerId doesn't match current version
  • Async processing: Returns 202 immediately, processes files in background
  • File handling: Downloads from URLs, auto-detects text vs binary, updates/creates/deletes files
  • Postback notifications: POSTs results (upToDate, outOfDate, invalidFiles, error) to git-bridge

• Security: Path traversal prevention, null byte checks, .git directory blocking

• Documentation: Updated README.md and USAGE.md with request/response formats and troubleshooting

Implementation

// Request from git-bridge
POST /api/v0/docs/:project_id/snapshots
{
  "latestVerId": 243,
  "files": [
    { "name": "main.tex", "url": "http://..." },  // modified
    { "name": "unchanged.tex" }                    // unchanged
  ],
  "postbackUrl": "http://git-bridge/postback"
}

// Immediate response
202 Accepted { "code": "accepted" }

// Async postback to git-bridge
POST {postbackUrl}
{ "code": "upToDate", "latestVerId": 244 }

Uses existing EditorController APIs (upsertDocWithPath, upsertFileWithPath, deleteEntityWithPath) for consistency with other project update paths.

Original prompt

#2 你虽然已经实现了git pull也就是读取数据的时候的逻辑，但是你还需要实现 Pushing a Project to WriteLaTeX Post 的逻辑 继续仔细阅读代码 然后帮我实现完所有的完整功能

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