docs: verify and complete Gateway CircuitBreaker updates in system hardening roadmap

Marked Gateway CircuitBreaker tasks as completed and verified in archived roadmaps and session logs. Confirmed fixes for `ClassNotFoundException` via Spring Cloud 2025.0.1 and proper configuration of Resilience4j with fallback and metrics integration.

Signed-off-by: Stefan Mogeritsch <stefan.mo.co@gmail.com>

docs/01_Architecture/_archive/2026-01-15_Roadmap_System_Hardening.md

@@ -20,6 +20,12 @@ last_update: 2026-03-09
     - `platform/build.gradle.kts`: BOM Import prüfen.
     - Ziel: Behebung der `ClassNotFoundException` im Gateway (CircuitBreaker).
     - _(verifiziert 2026-03-09)_
+- [x] **Gateway CircuitBreaker:** _(verifiziert 2026-03-09)_
+  - `spring-cloud-starter-circuitbreaker-reactor-resilience4j` (Reactive/WebFlux-Variante) korrekt eingebunden.
+  - `GatewayConfig.kt`: `circuitBreaker { name = "pingServiceCB"; fallbackUri = "forward:/fallback/ping" }`
+    konfiguriert.
+  - `FallbackController.kt` + `GatewayMetricsConfig.kt` (CB-Counter) vorhanden.
+  - Resilience4j `2.3.0` kompatibel mit Spring Cloud 2025.0.1 / Spring Boot 3.5.x.
 - [x] **Micrometer 1.16.1:**
     - Explizites Upgrade in `libs.versions.toml` für Java 25 Kompatibilität.
   - _(verifiziert 2026-03-09: micrometer = "1.16.1" bereits gesetzt, micrometerTracing = "1.6.1")_