feat(F-004): typed fail-fast config and feature flag module

- loadConfig: pure over env object, accumulates all problems, names var names only
- DATABASE_URL now required at startup; PORT/HOST/LOG_LEVEL/NODE_ENV/REDIS_URL defaulted
- flags module behind FeatureFlagProvider; unknown flags OFF; runtime setEnabled (no redeploy)
- buildApp decorates app.flags; server.ts fail-fast before app boot
- tests caught and fixed flag-store case-normalization bug before gates
- zero new dependencies; all gates approved; verify.sh green

project/README.md

@@ -19,6 +19,17 @@ npm run lint          # eslint + prettier check
 npm run lint:boundaries  # module boundary check
 ```
 
+## Configuration
+
+Startup is fail-fast: `src/infrastructure/config` parses env once and refuses to boot
+on missing/invalid required vars. `DATABASE_URL` is required; `PORT`, `HOST`,
+`LOG_LEVEL`, `NODE_ENV`, `REDIS_URL` are optional with defaults. Errors name variable
+NAMES only, never values.
+
+Feature flags: `FLAG_<NAME>=true|false` env vars seed the flag store at boot. Unknown
+flags default to OFF (fail-safe). Flags flip at runtime through the store — activation
+is separate from deployment (no redeploy). Copy `.env.example` to `.env` to start.
+
 ## HTTP contract
 
 - Every response carries an `x-request-id` header (propagated from a safe incoming