feat: P1 보안/성능 개선 및 마이그레이션 자동화

Security fixes:
- migrate.ts: SQL/Command Injection 방지 (spawnSync 사용)
- migrate.ts: Path Traversal 검증 추가
- api-tester.ts: API 키 마스킹 (4자만 노출)
- api-tester.ts: 최소 16자 키 길이 검증
- cache.ts: ReDoS 방지 (패턴 길이/와일드카드 제한)

Performance improvements:
- cache.ts: 순차 삭제 → 병렬 배치 처리 (50개씩)
- cache.ts: KV 등록 fire-and-forget (non-blocking)
- cache.ts: 메모리 제한 (5000키)
- cache.ts: 25초 실행 시간 가드
- cache.ts: 패턴 매칭 prefix 최적화

New features:
- 마이그레이션 자동화 시스템 (scripts/migrate.ts)
- KV 기반 캐시 인덱스 (invalidatePattern, clearAll)
- 글로벌 CacheService 싱글톤

Other:
- .env.example 추가, API 키 환경변수 처리
- CACHE_TTL.RECOMMENDATIONS (10분) 분리
- e2e-tester.ts JSON 파싱 에러 핸들링 개선

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

migrations/000_migration_history.sql

@@ -0,0 +1,34 @@
+-- Migration 000: Migration History Tracking
+-- Description: Creates table to track which migrations have been applied
+-- Date: 2026-01-25
+-- Author: Claude Code
+
+-- ============================================================
+-- Table: migration_history
+-- Purpose: Track applied database migrations
+-- ============================================================
+
+CREATE TABLE IF NOT EXISTS migration_history (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  migration_name TEXT NOT NULL UNIQUE,      -- e.g., "002_add_composite_indexes"
+  applied_at TEXT NOT NULL DEFAULT (datetime('now')),
+  execution_time_ms INTEGER,                -- Time taken to execute
+  success INTEGER NOT NULL DEFAULT 1,       -- 0=failed, 1=succeeded
+  error_message TEXT,                       -- Error details if failed
+  checksum TEXT                             -- Future: file content hash for validation
+);
+
+-- Index for quick lookup of applied migrations
+CREATE INDEX IF NOT EXISTS idx_migration_history_name
+  ON migration_history(migration_name);
+
+-- Index for status queries
+CREATE INDEX IF NOT EXISTS idx_migration_history_success
+  ON migration_history(success);
+
+-- ============================================================
+-- Notes
+-- ============================================================
+-- This table is created first (000) to track all subsequent migrations.
+-- Safe to re-run: uses IF NOT EXISTS for idempotency.
+-- Migrations are tracked by filename without .sql extension.