feat: implement POST /api/gists endpoint

[nullcoder]

Overview

Create the primary API endpoint for creating new encrypted gists, handling multipart form data and storing both metadata and encrypted blobs in R2.

Tasks

• Create POST /api/gists endpoint
• Implement multipart form data parsing (Workers-compatible)
• Add request size validation (Workers limit: 100MB)
• Validate input data structure
• Store metadata in R2 with proper key structure
• Store encrypted blob in R2
• Return gist ID and share URL

Technical Requirements

• Edge runtime compatible
• Parse multipart/form-data without Node.js dependencies
• Validate against GistMetadata schema
• Enforce size limits: 500KB per file, 5MB total, 20 files max
• Generate unique gist IDs with nanoid
• Atomic operation (rollback on failure)

Request Format

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Response Format

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Acceptance Criteria

• Endpoint accepts encrypted gist data
• Validates all required fields
• Enforces size limits
• Returns 201 with gist ID on success
• Returns 400 for validation errors
• Returns 413 for size limit exceeded
• Handles R2 errors appropriately

Dependencies

• feat: implement R2 storage operations #104 (Storage Operations)

References

• See docs/PHASE_5_ISSUE_TRACKING.md for detailed specifications