fix: un-ignore internal/build package

The build/ rule shadowed app/backend/internal/build; the !internal/build/
negation only matched the repo root. Package was never committed, breaking
CI backend tests and the Docker image build.

Authored By: TDvorak <[email protected]>

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

.gitignore

@@ -13,7 +13,7 @@ build/
 *.tsbuildinfo
 
 # Keep internal build system code
-!internal/build/
+!**/internal/build/
 
 # Environment variables
 .env