Completes master-cms-module: Build & Test, docs, and appsettings

Finishes the master-cms-module feature (Units 1-4): runs Build and
Test across master-backend, slave-availability-extension and
frontend-cms-page, fixes a missing Availability EF migration for
MasterRegistration and a TanStack Query v5 mutation-callback type
break, adds the missing MasterModule appsettings section, and
documents the module in README.md. Also seeds a tech-debt-backlog
feature to track dead config and pre-existing/introduced frontend
lint findings for later cleanup.

src/SlpModularCms.Api/appsettings.Development.json

@@ -20,6 +20,11 @@
     "CircuitBreakerSeconds": 30,
     "StatusCacheSeconds": 1
   },
+  "MasterModule": {
+    "IntegrityCheckIntervalMinutes": 60,
+    "HttpTimeoutSeconds": 10,
+    "MasterUrl": "https://localhost:7221"
+  },
   "Cors": {
     "AllowedOrigins": [
       "http://localhost:5173",

src/SlpModularCms.Api/appsettings.json

@@ -20,6 +20,11 @@
     "CircuitBreakerSeconds": 30,
     "StatusCacheSeconds": 1
   },
+  "MasterModule": {
+    "IntegrityCheckIntervalMinutes": 60,
+    "HttpTimeoutSeconds": 10,
+    "MasterUrl": "<public-url-of-this-master-instance>"
+  },
   "Cors": {
     "AllowedOrigins": []
   },