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_test.go

@@ -1,6 +1,7 @@
 package imageref
 
 import (
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -27,3 +28,19 @@ func TestParse(t *testing.T) {
 		})
 	}
 }
+
+func TestIsDigest(t *testing.T) {
+	cases := []struct {
+		in   string
+		want bool
+	}{
+		{"sha256:" + strings.Repeat("a", 64), true},
+		{"sha256:" + strings.Repeat("a", 63), false},
+		{"sha256:" + strings.Repeat("A", 64), false},
+		{"sha512:" + strings.Repeat("a", 64), false},
+		{"", false},
+	}
+	for _, c := range cases {
+		assert.Equal(t, c.want, IsDigest(c.in), c.in)
+	}
+}