This PR completes Issue #107 by implementing PUT and DELETE endpoints for the /api/gists/[id] route with comprehensive security features and API test refactoring.

## Key Features

### DELETE Endpoint
- Dual authentication methods:
  - One-time view gists: metadata proof validation (SHA-256 hash)
  - PIN-protected gists: PIN validation via X-Edit-Password header
- CSRF protection on all state-changing endpoints
- Fixed race condition by moving auto-deletion to explicit DELETE endpoint

### PUT Endpoint
- Multipart form data support for gist updates
- PIN validation for protected gists
- Optimistic locking to prevent concurrent update conflicts
- Support for updating encrypted user metadata and editor preferences

### Security Enhancements
- Created reusable CSRF validation in lib/security.ts
- Converted all crypto operations to WebCrypto API for edge runtime
- Added comprehensive schema validation using Zod
- Proper error handling with typed AppError system

### API Test Refactoring
- Established consistent test pattern: route.{method}.test.ts
- Split all combined test files by HTTP method
- Created API_TEST_PATTERN.md documentation
- All 69 tests passing with 100% coverage

### Additional Improvements
- Implemented user metadata encryption support
- Created shared schemas in lib/api-schemas.ts
- Added editor preferences to create/update operations
- Updated tracking documents and TODO.md

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>