Plan removal of obsolete legacy APIs

[cursor]

Problem

Legacy APIs remain in the codebase from the SQL Server-only era:

• [Obsolete] methods in DBTools/Core/DBTools.cs (connectDB, legacy retrieve methods)
• DBToolsController and DataExportController legacy controllers
• Properties/Settings.settings — likely unused WinForms-era config
• System.Configuration.ConfigurationManager package dependency

Related open issue: #12 (UtilsController/LinqHelper constructor breaking change)

Proposed solution

1. Inventory all [Obsolete] and legacy controller APIs
2. Document migration paths in README changelog
3. Set a target major version for removal
4. Remove unused Settings.settings if confirmed dead code
5. Evaluate whether ConfigurationManager dependency can be dropped

Acceptance criteria

• Obsolete API inventory documented
• Migration guide for each deprecated entry point
• Unused legacy files/dependencies identified for removal
• Removal scheduled with semver plan (coordinate with Configure NuGet package metadata and publishing workflow #25, Complete multi-provider naming migration (SqlClient and legacy artifacts) #32)