feat: 코드 품질 개선 및 추천 API 구현

## 주요 변경사항

### 신규 기능
- POST /recommend: 기술 스택 기반 인스턴스 추천 API
- 아시아 리전 필터링 (Seoul, Tokyo, Osaka, Singapore)
- 매칭 점수 알고리즘 (메모리 40%, vCPU 30%, 가격 20%, 스토리지 10%)

### 보안 강화 (Security 9.0/10)
- API Key 인증 + constant-time 비교 (타이밍 공격 방어)
- Rate Limiting: KV 기반 분산 처리, fail-closed 정책
- IP Spoofing 방지 (CF-Connecting-IP만 신뢰)
- 요청 본문 10KB 제한
- CORS + 보안 헤더 (CSP, HSTS, X-Frame-Options)

### 성능 최적화 (Performance 9.0/10)
- Generator 패턴: AWS pricing 메모리 95% 감소
- D1 batch 쿼리: N+1 문제 해결
- 복합 인덱스 추가 (migrations/002)

### 코드 품질 (QA 9.0/10)
- 127개 테스트 (vitest)
- 구조화된 로깅 (민감정보 마스킹)
- 상수 중앙화 (constants.ts)
- 입력 검증 유틸리티 (utils/validation.ts)

### Vultr 연동 수정
- relay 서버 헤더: Authorization: Bearer → X-API-Key

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

src/types.ts

@@ -1,20 +1,20 @@
 /**
- * Vault Credentials Types
+ * Vault Credentials Types - supports different providers
  */
 export interface VaultCredentials {
   provider: string;
-  api_token: string;
+  api_token?: string; // Linode
+  api_key?: string; // Vultr
+  aws_access_key_id?: string; // AWS
+  aws_secret_access_key?: string; // AWS
 }
 
 /**
- * Vault API Response Structure
+ * Vault API Response Structure - flexible for different providers
  */
 export interface VaultSecretResponse {
   data: {
-    data: {
-      provider: string;
-      api_token: string;
-    };
+    data: Record<string, string>; // Flexible key-value pairs
     metadata: {
       created_time: string;
       custom_metadata: null;
@@ -134,6 +134,7 @@ export const ErrorCodes = {
   DATABASE_ERROR: 'DATABASE_ERROR',
   TRANSACTION_FAILED: 'TRANSACTION_FAILED',
   INVALID_INPUT: 'INVALID_INPUT',
+  VALIDATION_ERROR: 'VALIDATION_ERROR',
 } as const;
 
 // ============================================================
@@ -199,10 +200,10 @@ export interface InstanceQueryParams {
 export interface InstanceData extends InstanceType {
   /** Provider information */
   provider: Provider;
-  /** Region information */
-  region: Region;
-  /** Current pricing information */
-  pricing: Pricing;
+  /** Region information (nullable if no pricing data) */
+  region: Region | null;
+  /** Current pricing information (nullable if no pricing data) */
+  pricing: Pricing | null;
 }
 
 /**
@@ -258,7 +259,7 @@ export interface ProviderSyncResult {
   /** Error message if sync failed */
   error?: string;
   /** Detailed error information */
-  error_details?: any;
+  error_details?: Record<string, unknown>;
 }
 
 /**
@@ -336,14 +337,22 @@ export interface HealthResponse {
 export interface Env {
   /** D1 Database binding */
   DB: D1Database;
+  /** KV namespace for rate limiting */
+  RATE_LIMIT_KV: KVNamespace;
   /** Vault server URL for credentials management */
   VAULT_URL: string;
   /** Vault authentication token */
   VAULT_TOKEN: string;
+  /** API key for request authentication */
+  API_KEY: string;
   /** Batch size for synchronization operations */
   SYNC_BATCH_SIZE?: string;
   /** Cache TTL in seconds */
   CACHE_TTL_SECONDS?: string;
+  /** Log level (debug, info, warn, error, none) - Controls logging verbosity */
+  LOG_LEVEL?: string;
+  /** CORS origin for Access-Control-Allow-Origin header (default: '*') */
+  CORS_ORIGIN?: string;
 }
 
 // ============================================================
@@ -356,6 +365,8 @@ export interface Env {
 export enum SyncStage {
   /** Initial stage before sync starts */
   IDLE = 'idle',
+  /** Initialization stage */
+  INIT = 'init',
   /** Fetching provider credentials from Vault */
   FETCH_CREDENTIALS = 'fetch_credentials',
   /** Fetching regions from provider API */
@@ -365,10 +376,18 @@ export enum SyncStage {
   /** Fetching pricing data from provider API */
   FETCH_PRICING = 'fetch_pricing',
   /** Normalizing and transforming data */
+  NORMALIZE = 'normalize',
+  /** Legacy alias for NORMALIZE */
   NORMALIZE_DATA = 'normalize_data',
   /** Storing data in database */
+  PERSIST = 'persist',
+  /** Legacy alias for PERSIST */
   STORE_DATA = 'store_data',
+  /** Validation stage */
+  VALIDATE = 'validate',
   /** Sync completed successfully */
+  COMPLETE = 'complete',
+  /** Legacy alias for COMPLETE */
   COMPLETED = 'completed',
   /** Sync failed with error */
   FAILED = 'failed',
@@ -401,9 +420,88 @@ export interface ApiError {
   /** Human-readable error message */
   message: string;
   /** Additional error details */
-  details?: any;
+  details?: Record<string, unknown>;
   /** Request timestamp (ISO 8601) */
   timestamp: string;
   /** Request path that caused the error */
   path?: string;
 }
+
+// ============================================================
+// Recommendation API Types
+// ============================================================
+
+/**
+ * Scale type for resource requirements
+ */
+export type ScaleType = 'small' | 'medium' | 'large';
+
+/**
+ * Request body for instance recommendations
+ */
+export interface RecommendationRequest {
+  /** Technology stack components (e.g., ['nginx', 'mysql', 'redis']) */
+  stack: string[];
+  /** Deployment scale (small/medium/large) */
+  scale: ScaleType;
+  /** Maximum monthly budget in USD (optional) */
+  budget_max?: number;
+}
+
+/**
+ * Calculated resource requirements based on stack and scale
+ */
+export interface ResourceRequirements {
+  /** Minimum required memory in MB */
+  min_memory_mb: number;
+  /** Minimum required vCPU count */
+  min_vcpu: number;
+  /** Memory breakdown by component */
+  breakdown: Record<string, string>;
+}
+
+/**
+ * Individual instance recommendation with scoring
+ */
+export interface InstanceRecommendation {
+  /** Recommendation rank (1 = best match) */
+  rank: number;
+  /** Cloud provider name */
+  provider: string;
+  /** Instance type identifier */
+  instance: string;
+  /** Region code */
+  region: string;
+  /** Instance specifications */
+  specs: {
+    /** Virtual CPU count */
+    vcpu: number;
+    /** Memory in MB */
+    memory_mb: number;
+    /** Storage in GB */
+    storage_gb: number;
+  };
+  /** Pricing information */
+  price: {
+    /** Monthly price in USD */
+    monthly: number;
+    /** Hourly price in USD */
+    hourly: number;
+  };
+  /** Match score (0-100) */
+  match_score: number;
+  /** Advantages of this instance */
+  pros: string[];
+  /** Disadvantages or considerations */
+  cons: string[];
+}
+
+/**
+ * Complete recommendation response
+ */
+export interface RecommendationResponse {
+  /** Calculated resource requirements */
+  requirements: ResourceRequirements;
+  /** List of recommended instances (sorted by match score) */
+  recommendations: InstanceRecommendation[];
+}