chore(canary): drop stray /healthcheck binary + ignore cmd build artifacts

go build ./cmd/healthcheck (run for verification, not via the bin/ recipe)
drops the binary in the working dir. Untracked previously; landed in the
last commit by accident. Gitignore now lists each cmd binary explicitly so
the same mistake can't recur for canary, healthcheck, or the two pdf/docx
template builders.

PROJECTS/beginner/canary-token-generator/.gitignore

@@ -24,6 +24,11 @@ backend/coverage.html
 backend/.golangci-cache/
 backend/data/
 backend/keys/
+# stray binaries from `go build ./cmd/<name>` run in backend/
+backend/canary
+backend/healthcheck
+backend/buildpdftemplate
+backend/builddocxtemplate
 
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 # Frontend (Node / Vite)