feat: 가벼운 자체 구현 Feature Flag 추가

[unam98]

작업 배경

Trunk-Based Development(#236)에서 "미완성 기능은 브랜치를 오래 살려두지 말고 feature flag로 감싸 main에 빠르게 합친다"를 실제로 가능하게 하는 최소 장치가 아직 없었다. SaaS(GrowthBook/LaunchDarkly류) 대신 가벼운 자체 구현을 선택했다.

변경 사항

영역	내용
config/featureflag/FeatureFlagProperties.java (신규)	feature-flags.flags.{key}: true/false 값을 Map<String, Boolean>으로 바인딩
config/featureflag/FeatureFlags.java (신규)	isEnabled(key) — yml에 없는 키는 기본값 false(비활성)로 처리
README.md	Feature Flag 사용법(yml 예시 + 코드 예시) 섹션 추가

영향 범위

• 아직 어떤 서비스 로직도 이 플래그를 참조하지 않음 — 순수 스캐폴딩, 런타임 동작 변화 없음
• yml에 feature-flags 섹션이 없어도 flags가 빈 Map으로 초기화되어 앱 구동에 영향 없음

검증 매트릭스

영향 범위	테스트 코드
활성화된 플래그 조회	• yml에_true로_설정된_플래그는_활성이다
비활성화된 플래그 조회	• yml에_false로_설정된_플래그는_비활성이다
정의되지 않은 키의 기본값	• yml에_정의되지_않은_플래그는_기본값으로_비활성이다

Test Plan

• 로컬 postgres/redis 기동 후 ./gradlew test 전체 254/254 통과

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📥 Commits

Reviewing files that changed from the base of the PR and between 8da4563 and 6b34a8b.

📒 Files selected for processing (4)

• README.md
• src/main/java/org/runnect/server/config/featureflag/FeatureFlagProperties.java
• src/main/java/org/runnect/server/config/featureflag/FeatureFlags.java
• src/test/java/org/runnect/server/config/featureflag/FeatureFlagsTest.java

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