Add file upload endpoint to OpenAPI specification

[c-st]

API documentation for #20

Add POST /v1/spaces/{space_id}/files endpoint for uploading files via multipart/form-data.
Includes FileUploadResponse schema with file_id (IPFS CID) and object_id fields.

Description

This PR adds OpenAPI documentation for the new file upload endpoint. The endpoint accepts
multipart/form-data file uploads and returns the created file object ID and IPFS CID.

Changes:

• Added POST /v1/spaces/{space_id}/files endpoint documentation
• Added apimodel.FileUploadResponse schema definition
• Includes proper request/response examples and error codes

What type of PR is this? (check all applicable)

• 🍕 Feature
• 🐛 Bug Fix
• 📝 Documentation Update
• 🎨 Style
• 🧑‍💻 Code Refactor
• 🔥 Performance Improvements
• ✅ Test
• 🤖 Build
• 🔁 CI

Related Tickets & Documents

• Implementation PR: [Link to anytype-heart PR once created]
• Adds REST API coverage for file upload functionality

Mobile & Desktop Screenshots/Recordings

N/A - OpenAPI specification update only.

Example usage:

curl -X POST http://localhost:31009/v1/spaces/{space_id}/files \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Anytype-Version: 2025-05-20" \
  -F "file=@/path/to/file.pdf"

Example response:
{
  "object_id": "bafyreiabc123def456",
  "file_id": "bafyreie6n5l5nkbjal37su54cha4coy7qzuhrnajluzv5qd5jvtsrxkequ",
  "details": {
    "name": "file.pdf",
    "sizeInBytes": 12345
  }
}

Added tests?

• 👍 yes

This is a documentation-only change. Tests are included in the implementation PR.

Added to documentation?

• 🙅 no documentation needed

The OpenAPI specification itself serves as the API documentation.