VACUUM snapshot metadata through the sqlite driver, not a CLI (closes #120)

snapshot create vacuumed the exported metadata database by shelling out
to a sqlite3 binary, so a backup failed at the very end on any host
without that CLI. vacuumDatabase now opens the database with the
modernc.org/sqlite driver and runs VACUUM through it, outside any
transaction; internal/snapshot no longer imports os/exec. The database
opens in WAL mode, so the close after VACUUM checkpoints the rewrite
into the main file that is compressed and uploaded. A new test deletes
marked rows, vacuums, and asserts the file shrank and no longer holds
the deleted bytes. script/bootstrap and the Dockerfile test image stop
installing the CLI.

Disclosure: the Dockerfile test-image change was not exercised by local
make check, which builds only the lint image.

Model: opus-4-8

Dockerfile

@@ -22,8 +22,10 @@ FROM golang:1.26.1-alpine@sha256:2389ebfa5b7f43eeafbd6be0c3700cc46690ef842ad962f
 
 ARG VERSION=dev
 
-# Install build dependencies for CGO (mattn/go-sqlite3) and sqlite3 CLI (tests)
-RUN apk add --no-cache make build-base sqlite
+# Build tooling: make, plus a C toolchain because `go test -race` needs cgo.
+# The sqlite driver is pure Go (modernc.org/sqlite), so no sqlite library or
+# CLI is required.
+RUN apk add --no-cache make build-base
 
 WORKDIR /src