Go tests run with -race and -cover under Go's own timeout (closes #88)

backend/script/test runs go test -timeout 30s -race -cover and, if
that fails, runs it again with -v and fails. The root script/test drops
its one 30-second timeout around both halves: from a cold Go build
cache, compiling the tests with -race used it all up on four cores.
Each half keeps its own limit. The builder stage of the Dockerfile
gains gcc and musl-dev for the race detector; make build still sets
CGO_ENABLED=0. New tests: the health check's answer, a valid report's
answer, a report file's exact lines, and the flush at the 10 MiB
threshold. The handlers TestImport stub is gone.

Model: opus-5-5

backend/internal/reportbuf/export_test.go

@@ -2,6 +2,10 @@ package reportbuf
 
 import "time"
 
+// FlushSizeThreshold exposes the buffer size at which Append starts
+// writing a report file to the external tests.
+const FlushSizeThreshold = flushSizeThreshold
+
 // Flush writes the buffered reports to a file now, as the periodic
 // flush does, so tests need not wait a minute for it.
 func (b *Buffer) Flush() error {