refactor: code review 기반 품질 개선

- HonoVariables 타입 중앙화 (types.ts로 추출, 5개 파일 중복 제거)
- 6시간 pricing update cron 핸들러 추가 (syncPricingOnly 메서드)
- Response.json() → c.json() 패턴 통일 (Hono 표준)
- SORT_FIELD_MAP 중앙화 (constants.ts, 12개 필드 지원)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/constants.ts

@@ -129,22 +129,32 @@ export const TABLES = {
 // ============================================================
 
 /**
- * Valid sort fields for instance queries
+ * Mapping of user-facing sort field names to database column names
+ *
+ * This is the single source of truth for sort field validation and mapping.
+ * Query aliases: it=instance_types, pr=pricing, p=providers, r=regions
  */
-export const VALID_SORT_FIELDS = [
-  'price',
-  'hourly_price',
-  'monthly_price',
-  'vcpu',
-  'memory_mb',
-  'memory_gb',
-  'storage_gb',
-  'instance_name',
-  'provider',
-  'region',
-] as const;
+export const SORT_FIELD_MAP: Record<string, string> = {
+  price: 'pr.hourly_price',
+  hourly_price: 'pr.hourly_price',
+  monthly_price: 'pr.monthly_price',
+  vcpu: 'it.vcpu',
+  memory: 'it.memory_mb',
+  memory_mb: 'it.memory_mb',
+  memory_gb: 'it.memory_mb', // Note: memory_gb is converted to memory_mb at query level
+  storage_gb: 'it.storage_gb',
+  name: 'it.instance_name',
+  instance_name: 'it.instance_name',
+  provider: 'p.name',
+  region: 'r.region_code',
+} as const;
 
-export type ValidSortField = typeof VALID_SORT_FIELDS[number];
+/**
+ * Valid sort fields for instance queries (derived from SORT_FIELD_MAP)
+ */
+export const VALID_SORT_FIELDS = Object.keys(SORT_FIELD_MAP) as ReadonlyArray<string>;
+
+export type ValidSortField = keyof typeof SORT_FIELD_MAP;
 
 /**
  * Valid sort orders