feat: enforce tenant isolation and shared question bank

Tiku.Infrastructure/Tenancy/TenantDirectory.cs

@@ -0,0 +1,66 @@
+using Npgsql;
+using Tiku.Application.Tenancy;
+using Tiku.Domain.Tenancy;
+
+namespace Tiku.Infrastructure.Tenancy;
+
+public sealed class TenantDirectory(NpgsqlDataSource dataSource) : ITenantDirectory
+{
+    public Task<TenantDirectoryEntry?> FindByHostAsync(
+        string host,
+        CancellationToken cancellationToken = default)
+    {
+        const string sql = """
+            select t.id, t.slug, t.name, t.status, t.mode, d.host
+            from tenant_domains d
+            join tenants t on t.id = d.tenant_id
+            where d.host = @lookup
+              and d.status = 'active'
+              and t.status = 'active'
+            limit 1
+            """;
+        return FindAsync(sql, host, cancellationToken);
+    }
+
+    public Task<TenantDirectoryEntry?> FindByCodeAsync(
+        string tenantCode,
+        CancellationToken cancellationToken = default)
+    {
+        const string sql = """
+            select t.id, t.slug, t.name, t.status, t.mode, null::text as host
+            from tenants t
+            where t.slug = @lookup
+              and t.status = 'active'
+            limit 1
+            """;
+        return FindAsync(sql, tenantCode, cancellationToken);
+    }
+
+    private async Task<TenantDirectoryEntry?> FindAsync(
+        string sql,
+        string lookup,
+        CancellationToken cancellationToken)
+    {
+        await using var command = dataSource.CreateCommand(sql);
+        command.Parameters.AddWithValue("lookup", lookup);
+        await using var reader = await command.ExecuteReaderAsync(cancellationToken);
+        if (!await reader.ReadAsync(cancellationToken))
+        {
+            return null;
+        }
+
+        return new TenantDirectoryEntry(
+            reader.GetGuid(0),
+            reader.GetString(1),
+            reader.GetString(2),
+            ParseEnum<TenantStatus>(reader.GetString(3)),
+            ParseEnum<TenantMode>(reader.GetString(4)),
+            reader.IsDBNull(5) ? null : reader.GetString(5));
+    }
+
+    private static TEnum ParseEnum<TEnum>(string value)
+        where TEnum : struct, Enum
+    {
+        return Enum.Parse<TEnum>(value.Replace("_", string.Empty, StringComparison.Ordinal), true);
+    }
+}