TASKS — F-004 Typed config and feature flags

TASK-001 src/infrastructure/config/config.ts: AppConfig, ConfigError, loadConfig (pure over env object)
TASK-002 src/infrastructure/config/tests/config.test.ts: required/optional/invalid cases
TASK-003 src/modules/flags/domain/feature-flag-store.ts: FeatureFlagProvider + InMemoryFeatureFlagStore + createFlagStore
TASK-004 src/modules/flags/index.ts: public API
TASK-005 src/modules/flags/tests/feature-flag-store.test.ts: guard + runtime mutation
TASK-006 src/app/build-app.ts: optional flags dep
TASK-007 src/infrastructure/http/server.ts: loadConfig, typed listen, ConfigError exit
TASK-008 run lint + boundaries + typecheck + build + test + verify.sh