docs: Correct Dockerfile location and specify the verify probe

vantagectl/Dockerfile follows the repo's per-module convention rather than
living under deploy/docker.

verify's live check needs AES-GCM open, so the cipher primitives move to
shared/cryptobox and services/crypto.go delegates, rather than a second
copy of the cipher existing in another module.

docs/superpowers/specs/2026-09-07-control-plane-backup-restore-design.md

@@ -223,9 +223,21 @@ fingerprint — and contacts no database. It is what an operator runs to find ou
 whether an archive they have found is worth anything.
 
 `verify` adds a live check: whether the archive's fingerprint matches the key in
-the current environment, and whether it matches the database being pointed at.
-This is the command that distinguishes "we have backups" from "we have backups
-that will restore", and the documentation recommends running it on a schedule.
+the current environment, and — when `--mongo-uri` is given — whether that key
+actually decrypts the target database. The second half is a probe: read one
+ciphertext field from `secrets`, `keys` or `auth_providers` and attempt to open
+it. A fingerprint comparison proves two archives agree; only a probe proves the
+key in hand opens the data in front of you. `verify` is the command that
+distinguishes "we have backups" from "we have backups that will restore", and
+the documentation recommends running it on a schedule.
+
+The probe needs AES-256-GCM open, which today lives in
+`server/internal/services/crypto.go` and cannot be imported from another module.
+Rather than copy it — the exact hazard `CLAUDE.md` names around mirrored token
+blocks and `web/lib/targets.ts` — the primitives move to a new `shared/cryptobox`
+package, and `services/crypto.go` becomes a thin delegation that keeps its
+existing unexported function names and its `KEY_ENCRYPTION_KEY` lookup. One
+implementation of the cipher, two callers.
 
 `--out -` streams the tarball to stdout, so piping into `aws s3 cp -`, `restic`
 or `age` covers storage and archive encryption without the tool growing backends
@@ -241,7 +253,9 @@ Kubernetes, or neither.
 `linux/arm64`, `darwin/arm64` and `windows/amd64` with `CGO_ENABLED=0`, writes
 `checksums.txt`, and creates a Gitea release.
 
-**Container image.** `deploy/docker/vantagectl.Dockerfile` produces a `scratch`
+**Container image.** `vantagectl/Dockerfile` — the repo's convention is a
+Dockerfile per module built from the repository root, because every Go module
+depends on `shared` through a replace directive — produces a `scratch`
 image holding the static binary and an explicitly copied `/tmp`, which the
 archive is staged in before compression — the same omission that silently
 disabled `vulnsched` on a scratch image. Pushed by `server-deploy.yml` as an