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

script/bootstrap

@@ -114,9 +114,6 @@ main() {
     # from CI. Nothing on the host is ever used as a linter, at any
     # version, so installing one here would buy nothing.
 
-    # sqlite3 CLI: the test suite shells out to it (VACUUM).
-    if missing sqlite3; then pkg_install sqlite sqlite3 sqlite sqlite; fi
-
     # goreleaser, at the version pinned by script/install-goreleaser and
     # verified against a hardcoded sha256. Package managers are not used
     # for it: they ship whatever version they happen to carry, and the