feat: master/agent phase 5 — drift and merge, backend (#10) (#30)

## Summary
- Drift detection: `model.Service.Drifted()` compares desired and running version by digest (tag when the digest is unknown). `services.awaiting_inventory` hides drift between a job result and the next inventory write.
- Poll and webhook jobs are refused for a drifted service; `POST /services/{id}/restore` (manual job to `desired_ref`) and `POST /services/{id}/accept` (running version becomes desired, guarded by `actual` = `Service.RunningRef`).
- Move suggestions (`store.MoveCandidates`, Compose only) and `POST /services/{id}/merge`: the old row keeps its id, mode, `desired_ref` and history and takes the new location; the duplicate is deleted.
- Backend only. The buttons and badges come with #28, on top of the Ledger UI (#18).

Closes #10

## Test plan
- [x] `gofmt -l .`, `go vet ./...`, `go test ./...`
- [x] `go test -race ./internal/agent/... ./internal/master/... ./internal/e2e/`
- [x] `go test -tags integration ./internal/master/store/` against postgres:17-alpine
- [ ] On the server: migration 0005 applies; no drift shown after a normal update
- [ ] On the server: manual `docker compose up` with another tag gives drift; webhook for an auto service is refused
- [ ] On the server: restore and accept via POST; merge after moving a stack

Reviewed-on: #30

internal/imageref/imageref.go

@@ -2,7 +2,10 @@
 // and digest.
 package imageref
 
-import "strings"
+import (
+	"regexp"
+	"strings"
+)
 
 // Ref is an image reference split into its parts. Tag and Digest are empty
 // when the reference has none.
@@ -41,3 +44,8 @@ func (r Ref) String() string {
 	}
 	return s
 }
+
+var digestPattern = regexp.MustCompile(`^sha256:[0-9a-f]{64}$`)
+
+// IsDigest reports whether d is a well-formed sha256 digest.
+func IsDigest(d string) bool { return digestPattern.MatchString(d) }