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/README.md

@@ -35,7 +35,10 @@ pattern as the repo root: the targets in `backend/Makefile` are thin shims over
   stamped in. The version is `VERSION` from the environment;
   when that is unset or empty, it falls back to `git describe` inside a git
   checkout, then to `dev`
-- `script/test` — run the Go tests under a 30-second timeout
+- `script/test` — run the Go tests with the race detector and coverage. Go's
+  `-timeout 30s` bounds the tests, not their compile. If they fail, they run
+  again with `-v` for the details, and the script fails. The race detector needs
+  a C compiler
 - `script/lint` — check `.golangci.yml` against its pinned sha256, then run
   golangci-lint. It runs inside the golangci-lint image of the lint stage of
   the root `Dockerfile`; from a checkout, run `make lint` at the repo root,