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

snapshot create compacted the metadata database by running a sqlite3 command-line binary, after every blob had already been uploaded. On a host without that binary, which includes anyone who installed with go install, the backup failed at the last step, and two tests failed the same way.

VACUUM now runs through the Go sqlite driver the program already uses, and its error is returned to the caller. The runtime Docker image no longer installs the sqlite package, since nothing in the binary calls it.

model: claude-opus-4-8 (implementation, review); claude-fable-5-1 (merge)

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
 
@@ -71,7 +73,7 @@ RUN CGO_ENABLED=0 go build -ldflags "-X 'sneak.berlin/go/vaultik/internal/global
 # alpine:3.21, 2026-02-25
 FROM alpine:3.21@sha256:c3f8e73fdb79deaebaa2037150150191b9dcbfba68b4a46d70103204c53f4709
 
-RUN apk add --no-cache ca-certificates sqlite
+RUN apk add --no-cache ca-certificates
 
 # Copy binary from builder
 COPY --from=builder /vaultik /usr/local/bin/vaultik