docs: update documentation for extraction functions migration

- Add ExtractionFunctions.md reference document
- Update database-schema spec with 11 extraction functions
- Update data-access spec to document extraction function approach
- Update search-processing spec with new query builder interface
- Add Database.Tests to Testing.md architecture doc
- Update DataFlow.md with extraction function flow

DOCUMENTATION/Architecture/DataFlow.md

@@ -19,6 +19,8 @@ The search flow mirrors the legacy pattern, modernized for ASP.NET Core:
 
 4. SearchProcessorService (BackgroundService) polls
    └─> Finds queued searches
+   └─> Passes SearchId to query builder
+   └─> SQL extraction functions read criteria from Search.Criteria JSON
    └─> Executes query against local cache
    └─> Generates Excel via ClosedXML
    └─> Stores result in Search.Results (VARBINARY)