chore(build, docs): add security module and update infrastructure decisions

- Created `backend/infrastructure/security` module with standardized configurations for OAuth2, JWT validation, CORS, and role mapping.
- Updated ADRs to reflect resolved backend infrastructure decisions, including security standardization, persistence strategy, and Flyway schema location.
- Enabled integration of the `security` module into relevant projects (e.g., `ping-service` and `gateway`).

settings.gradle.kts

@@ -61,6 +61,9 @@ include(":backend:infrastructure:monitoring:monitoring-server")
 // --- PERSISTENCE ---
 include(":backend:infrastructure:persistence")
 
+// --- SECURITY ---
+include(":backend:infrastructure:security")
+
 // === BACKEND - SERVICES ===
 // --- ENTRIES (Nennungen) ---
 include(":backend:services:entries:entries-api")