refactor: address code review findings across all projects

Apply comprehensive fixes from code reviews including:
- Extract shared utilities (SqlFormatHelper, CellValueConverter, DbDestinationBase)
- Add interface abstractions (IAuthenticationService, IDatabaseMigrator, IMisQueryBuilder)
- Implement SecureStore for encrypted secrets storage
- Fix error handling with proper HTTP status codes and logging
- Optimize double enumeration in DevEtlRegistry
- Add DataSync.Dev README for developer onboarding
- Extract filter panel base classes to reduce duplication
- Update code review docs to mark all issues as fixed

NEW/src/JdeScoping.Core/Interfaces/IAuthenticationService.cs

@@ -0,0 +1,22 @@
+using JdeScoping.Core.Models;
+
+namespace JdeScoping.Core.Interfaces;
+
+/// <summary>
+/// Core authentication service interface.
+/// Provides credential-based user authentication.
+/// </summary>
+public interface IAuthenticationService
+{
+    /// <summary>
+    /// Authenticates a user with the given credentials.
+    /// </summary>
+    /// <param name="username">Username</param>
+    /// <param name="password">Password</param>
+    /// <param name="ct">Cancellation token</param>
+    /// <returns>Authentication result containing success status and user info if successful</returns>
+    Task<AuthResult> AuthenticateAsync(
+        string username,
+        string password,
+        CancellationToken ct = default);
+}