feat(configdb): add SiteCalls migration (#22, #23 M3)

Bundle B2 of Audit Log #23 M3: EF-generated migration that creates the
SiteCalls operational-state table on [PRIMARY], with the simple clustered
PK on TrackedOperationId and the two named indexes the entity config
declares.

No partition function / scheme / DB-role restriction — SiteCalls holds
mutable operational state (insert-once + monotonic-status update at the
repo layer), unlike the partitioned append-only AuditLog table from M1.

- Migration: 20260520180431_AddSiteCallsTable.cs (auto-generated;
  EF emitted CREATE TABLE + 2 indexes without customisation needed).
- Model snapshot updated alongside.
- Integration test: tests/ScadaLink.ConfigurationDatabase.Tests/Migrations/
  AddSiteCallsTableMigrationTests.cs. Uses the existing MsSqlMigrationFixture
  with [SkippableFact] + Skip.IfNot(fixture.Available). Asserts table +
  twelve columns + PK on TrackedOperationId + both named indexes.

tests/ScadaLink.ConfigurationDatabase.Tests/Migrations/AddSiteCallsTableMigrationTests.cs

@@ -0,0 +1,125 @@
+using Microsoft.Data.SqlClient;
+using Xunit;
+
+namespace ScadaLink.ConfigurationDatabase.Tests.Migrations;
+
+/// <summary>
+/// Bundle B2 (#22, #23 M3) integration tests for the <c>AddSiteCallsTable</c>
+/// migration: applies the EF migrations to a freshly-created MSSQL test database
+/// on the running infra/mssql container and asserts that the resulting
+/// <c>SiteCalls</c> table carries the expected columns, primary key, and the
+/// two named operational indexes.
+/// </summary>
+/// <remarks>
+/// Unlike <c>AddAuditLogTable</c>, the SiteCalls table is operational (mutable)
+/// state — no partition function, no partition scheme, no DB-role restriction.
+/// Standard <c>[PRIMARY]</c> filegroup. Tests pair <see cref="SkippableFactAttribute"/>
+/// with <c>Skip.IfNot(...)</c> so the runner reports them as Skipped (not Passed)
+/// when MSSQL is unreachable. The fixture applies the migration once at
+/// construction time.
+/// </remarks>
+public class AddSiteCallsTableMigrationTests : IClassFixture<MsSqlMigrationFixture>
+{
+    private readonly MsSqlMigrationFixture _fixture;
+
+    public AddSiteCallsTableMigrationTests(MsSqlMigrationFixture fixture)
+    {
+        _fixture = fixture;
+    }
+
+    [SkippableFact]
+    public async Task AppliesMigration_CreatesSiteCallsTable_WithExpectedColumns()
+    {
+        Skip.IfNot(_fixture.Available, _fixture.SkipReason);
+
+        var exists = await ScalarAsync<int>(
+            "SELECT COUNT(*) FROM INFORMATION_SCHEMA.TABLES " +
+            "WHERE TABLE_NAME = 'SiteCalls' AND TABLE_SCHEMA = 'dbo';");
+        Assert.Equal(1, exists);
+
+        // Every required column from SiteCall + IngestedAtUtc. We don't pin types
+        // here because EF's CreateTable layer already encodes them; the
+        // entity-config tests cover length / unicode / nullability for the
+        // value-converted PK column. Just confirm the schema has all twelve.
+        var expectedColumns = new[]
+        {
+            "TrackedOperationId",
+            "Channel",
+            "Target",
+            "SourceSite",
+            "Status",
+            "RetryCount",
+            "LastError",
+            "HttpStatus",
+            "CreatedAtUtc",
+            "UpdatedAtUtc",
+            "TerminalAtUtc",
+            "IngestedAtUtc",
+        };
+
+        foreach (var column in expectedColumns)
+        {
+            var present = await ScalarAsync<int>(
+                "SELECT COUNT(*) FROM INFORMATION_SCHEMA.COLUMNS " +
+                $"WHERE TABLE_NAME = 'SiteCalls' AND COLUMN_NAME = '{column}';");
+            Assert.True(present == 1, $"Expected SiteCalls.{column} to exist; found {present}.");
+        }
+    }
+
+    [SkippableFact]
+    public async Task AppliesMigration_CreatesPK_OnTrackedOperationId()
+    {
+        Skip.IfNot(_fixture.Available, _fixture.SkipReason);
+
+        // Walk sys.indexes for the table's clustered PK index and confirm its
+        // single key column is TrackedOperationId. SiteCalls is non-partitioned
+        // so the PK is a simple single-column clustered index.
+        var pkColumn = await ScalarAsync<string?>(
+            "SELECT c.name FROM sys.indexes i " +
+            "INNER JOIN sys.objects o ON i.object_id = o.object_id " +
+            "INNER JOIN sys.index_columns ic ON i.object_id = ic.object_id AND i.index_id = ic.index_id " +
+            "INNER JOIN sys.columns c ON ic.object_id = c.object_id AND ic.column_id = c.column_id " +
+            "WHERE o.name = 'SiteCalls' AND i.is_primary_key = 1;");
+
+        Assert.Equal("TrackedOperationId", pkColumn);
+    }
+
+    [SkippableFact]
+    public async Task AppliesMigration_CreatesIndex_Source_Created()
+    {
+        Skip.IfNot(_fixture.Available, _fixture.SkipReason);
+
+        var count = await ScalarAsync<int>(
+            "SELECT COUNT(*) FROM sys.indexes i " +
+            "INNER JOIN sys.objects o ON i.object_id = o.object_id " +
+            "WHERE o.name = 'SiteCalls' AND i.name = 'IX_SiteCalls_Source_Created';");
+        Assert.Equal(1, count);
+    }
+
+    [SkippableFact]
+    public async Task AppliesMigration_CreatesIndex_Status_Updated()
+    {
+        Skip.IfNot(_fixture.Available, _fixture.SkipReason);
+
+        var count = await ScalarAsync<int>(
+            "SELECT COUNT(*) FROM sys.indexes i " +
+            "INNER JOIN sys.objects o ON i.object_id = o.object_id " +
+            "WHERE o.name = 'SiteCalls' AND i.name = 'IX_SiteCalls_Status_Updated';");
+        Assert.Equal(1, count);
+    }
+
+    // --- helpers ------------------------------------------------------------
+
+    private async Task<T> ScalarAsync<T>(string sql)
+    {
+        await using var conn = _fixture.OpenConnection();
+        await using var cmd = conn.CreateCommand();
+        cmd.CommandText = sql;
+        var result = await cmd.ExecuteScalarAsync();
+        if (result is null || result is DBNull)
+        {
+            return default!;
+        }
+        return (T)Convert.ChangeType(result, typeof(T) == typeof(string) ? typeof(string) : Nullable.GetUnderlyingType(typeof(T)) ?? typeof(T))!;
+    }
+}