Phase 0 WP-0.2–0.9: Implement Commons (types, entities, interfaces, messages, protocol, tests)

- WP-0.2: Namespace/folder skeleton (26 directories)
- WP-0.3: Shared data types (6 enums, RetryPolicy, Result<T>)
- WP-0.4: 24 domain entity POCOs across 10 domain areas
- WP-0.5: 7 repository interfaces with full CRUD signatures
- WP-0.6: IAuditService cross-cutting interface
- WP-0.7: 26 message contract records across 8 concern areas
- WP-0.8: IDataConnection protocol abstraction with batch ops
- WP-0.9: 8 architectural constraint enforcement tests
All 40 tests pass, zero warnings.

src/ScadaLink.Commons/Entities/ExternalSystems/DatabaseConnectionDefinition.cs

@@ -0,0 +1,16 @@
+namespace ScadaLink.Commons.Entities.ExternalSystems;
+
+public class DatabaseConnectionDefinition
+{
+    public int Id { get; set; }
+    public string Name { get; set; }
+    public string ConnectionString { get; set; }
+    public int MaxRetries { get; set; }
+    public TimeSpan RetryDelay { get; set; }
+
+    public DatabaseConnectionDefinition(string name, string connectionString)
+    {
+        Name = name ?? throw new ArgumentNullException(nameof(name));
+        ConnectionString = connectionString ?? throw new ArgumentNullException(nameof(connectionString));
+    }
+}