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. Each half keeps
its own limit. The race detector needs a C compiler: the Dockerfile
builder stage gains gcc and musl-dev, and script/bootstrap installs gcc,
with the C library headers on apt and apk, when gcc is missing; 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

README.md

@@ -39,14 +39,16 @@ halves, so the root `make check` fails if either one is broken. We provide:
 - `script/bootstrap` — install all dependencies (the pinned node via nvm unless
   one new enough for the frontend's dependencies is installed, yarn via
   corepack, `yarn install --frozen-lockfile`, the pinned Go unless one at least
-  as new as `backend/go.mod` asks for is installed, and the Go modules), linking
-  what it installs itself into `~/.local/bin`, which has to be on `PATH`. It
-  installs no Go linter and not Docker: `make lint` runs the linter in Docker
+  as new as `backend/go.mod` asks for is installed, the Go modules, and gcc with
+  the C library headers unless gcc is installed, for the race detector in
+  `make test`), linking what it installs itself into `~/.local/bin`, which has
+  to be on `PATH`. It installs no Go linter and not Docker: `make lint` runs the
+  linter in Docker
 - `script/setup` — make a fresh clone ready for development: bootstrap plus the
   git pre-commit hook
 - `script/projectname` — print the project name (used for the Docker image tag)
-- `script/test` — run `script/frontend-test`, then the backend's Go tests, both
-  within one 30-second timeout
+- `script/test` — run `script/frontend-test`, then the backend's Go tests, each
+  under its own 30-second timeout
 - `script/lint` — run `script/frontend-lint`, then golangci-lint in Docker, by
   building the lint stage of `Dockerfile` without the cache
 - `script/fmt` — format all files (writes): prettier, then gofmt over `backend/`