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

work/artifacts/F-004/documenter.md

@@ -0,0 +1,19 @@
+# Documenter — F-004 Typed config and feature flags
+
+done -> work/artifacts/F-004/documenter.md
+
+## Why documentation applies
+F-004 changes the startup contract (DATABASE_URL now required, fail-fast behavior) and introduces the flag mechanism every future risky feature will use.
+
+## Documents reviewed/produced
+- `project/README.md`: "Configuration" section added at build stage — required/optional vars, fail-fast behavior, FLAG_* semantics, fail-safe defaults, activation ≠ deployment.
+- `.env.example`: REDIS_URL + FLAG_EXAMPLE_FEATURE added with explanatory comments.
+- `spec/tech.md`: no new dependency rows needed — ticket added zero dependencies by design (noted for audit trail).
+- `specs/F-004-config-flags/` SPEC/DESIGN/TASKS/TESTS consistent with implementation.
+
+## Verification
+- README Configuration section cross-checked against config.ts behavior (required DATABASE_URL; PORT/HOST/LOG_LEVEL/NODE_ENV/REDIS_URL defaults; FLAG_* parsing).
+- Startup contract change (DATABASE_URL required) is called out explicitly so operators are not surprised.
+
+## Pending docs (not this ticket)
+- Flag admin transport documented when first consumer lands.