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.

aidlc-docs/features/master-cms-module/construction/build-and-test/unit-test-instructions.md

@@ -0,0 +1,33 @@
+# Unit Test Execution
+
+## Run Backend Unit Tests
+```bash
+dotnet test SlpModularCms.sln --no-build
+```
+
+### Expected Results
+| Test Project | Tests | Result |
+|---|---|---|
+| SlpModularCms.Core.Tests | 45 | Pass |
+| SlpModularCms.Modules.Identity.Tests | 46 | Pass |
+| SlpModularCms.Modules.Availability.Tests | 60 | Pass |
+| SlpModularCms.Modules.Master.Tests | 42 | Pass |
+| **Total** | **193** | **All Pass** |
+
+## Run Frontend Unit Tests
+```bash
+cd frontend
+npm run test -- --run
+```
+
+### Expected Results
+- **Test Files**: 34 passed (34)
+- **Tests**: 209 passed (209)
+- `Not implemented: Window's scrollTo()` messages are jsdom environment noise, not failures
+
+## Fix Failing Tests
+If tests fail:
+1. Review console output for the failing test name and assertion
+2. For backend: check the relevant module's test project under `src/`
+3. For frontend: check `frontend/src/**/*.test.ts(x)`
+4. Rerun the specific test project/file until green