feat(api-key): 添加 IP 白名单/黑名单限制功能 (#221)

* 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

* fix: update API contract tests for ip_whitelist/ip_blacklist fields

Add ip_whitelist and ip_blacklist fields to expected JSON responses
in API contract tests to match the new API key schema.

backend/internal/server/api_contract_test.go

@@ -82,6 +82,8 @@ func TestAPIContracts(t *testing.T) {
 					"name": "Key One",
 					"group_id": null,
 					"status": "active",
+					"ip_whitelist": null,
+					"ip_blacklist": null,
 					"created_at": "2025-01-02T03:04:05Z",
 					"updated_at": "2025-01-02T03:04:05Z"
 				}
@@ -116,6 +118,8 @@ func TestAPIContracts(t *testing.T) {
 							"name": "Key One",
 							"group_id": null,
 							"status": "active",
+							"ip_whitelist": null,
+							"ip_blacklist": null,
 							"created_at": "2025-01-02T03:04:05Z",
 							"updated_at": "2025-01-02T03:04:05Z"
 						}