Adds a MariaDB service to backend-test for SlpModularCms.Api.Tests

The new pipeline regression tests boot the real Api host via
WebApplicationFactory, which unconditionally runs the startup database
migration -- unlike every other test project here, which mocks or uses EF
Core InMemory. CI had no MariaDB service at all, so those tests failed on
the connection itself. Adds a mariadb service container to the backend-test
job plus a ConnectionStrings__DefaultConnection override for that step,
which overrides the appsettings.Development.json placeholder via standard
config layering without touching any committed appsettings file or
affecting local test runs.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FWyStNL2ZsjrS7FLd7xvvN

.gitea/workflows/continuous_integration.yaml

@@ -128,12 +128,37 @@ jobs:
     needs: [changes, backend-build]
     if: needs.changes.outputs.backend == 'true'
     runs-on: ubuntu-latest
+    # Added for SlpModularCms.Api.Tests (slpsoftware-api feature, NFR-CS-01): its pipeline
+    # regression tests boot the real Api host via WebApplicationFactory, which unconditionally
+    # runs CmsHost.ConfigurePipeline's startup migration (MigrateCoreDatabase) -- unlike every
+    # other test project here, which mocks/uses EF Core InMemory and never touches a real
+    # database. Without a real, reachable MariaDB, those tests fail on the connection itself
+    # before any assertion runs.
+    services:
+      mariadb:
+        image: mariadb:11
+        env:
+          MARIADB_ROOT_PASSWORD: ci_test_password
+          MARIADB_DATABASE: SlpModularCms
+        ports:
+          - 3306:3306
+        options: >-
+          --health-cmd="mysqladmin ping --silent"
+          --health-interval=10s
+          --health-timeout=5s
+          --health-retries=5
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-dotnet@v4
         with:
           dotnet-version: ${{ env.DOTNET_VERSION }}
       - name: Test (Release)
+        env:
+          # Overrides the placeholder in SlpModularCms.Api/appsettings.Development.json for this
+          # CI run only -- ASP.NET Core's configuration layering applies environment variables
+          # after appsettings.*.json, so this reaches SlpModularCms.Api.Tests' WebApplicationFactory
+          # without touching any committed appsettings file.
+          ConnectionStrings__DefaultConnection: "Server=127.0.0.1;Port=3306;Database=SlpModularCms;Uid=root;Pwd=ci_test_password"
         run: dotnet test SlpModularCms.sln -c Release
 
   # --- Gate 3: vulnerability scan ---