Security & Validation

[lephorx]

Title: Add API key authentication system

Description:
Implement API key-based authentication to secure endpoints and track usage.

Tasks:

• Create API key generation endpoint
• Add middleware to validate API keys
• Store API keys securely (database/env)
• Add rate limiting per API key
• Create API key management endpoints (list, revoke, regenerate)
• Add authentication to sensitive endpoints
• Update API documentation with auth examples

Acceptance Criteria:

• All conversion endpoints require valid API key
• Invalid/missing keys return 401 Unauthorized
• API keys can be managed via admin endpoints
• Rate limiting prevents abuse