feat(api-key): add IP whitelist/blacklist restriction and usage log IP tracking

- Add IP restriction feature for API keys (whitelist/blacklist with CIDR support)
- Add IP address logging to usage logs (admin-only visibility)
- Remove billing_type column from usage logs UI (redundant)
- Use generic "Access denied" error message for security

Backend:
- New ip package with IP/CIDR validation and matching utilities
- Database migrations for ip_whitelist, ip_blacklist (api_keys) and ip_address (usage_logs)
- Middleware IP restriction check after API key validation
- Input validation for IP/CIDR patterns on create/update

Frontend:
- API key form with enable toggle for IP restriction
- Shield icon indicator in table for keys with IP restriction
- Removed billing_type filter and column from usage views

backend/migrations/031_add_ip_address.sql

@@ -0,0 +1,5 @@
+-- Add IP address field to usage_logs table for request tracking (admin-only visibility)
+ALTER TABLE usage_logs ADD COLUMN IF NOT EXISTS ip_address VARCHAR(45);
+
+-- Create index for IP address queries
+CREATE INDEX IF NOT EXISTS idx_usage_logs_ip_address ON usage_logs(ip_address);