P3-09 + P3-X3: snapshot diff + recent-restores line

P3-09 — snapshot diff dispatcher.
- POST /api/hosts/{id}/snapshots/diff (and the unprefixed HTMX-form
  variant) takes {snapshot_a, snapshot_b}, validates both belong to
  the host (long id / short id / prefix match), checks the agent is
  online, mints a JobDiff, ships command.run with DiffPayload, writes
  a host.snapshot_diff audit row, returns HX-Redirect to the live
  job page (or JSON {job_id, job_url} for REST callers).
- Two-snapshot guard: POSTing diff(a,a) returns 422.
- UI: small panel on the host_detail right rail (visible when the
  host has 2+ snapshots) with two short-id inputs and a Diff button.
  Output renders on the standard live job page where the operator
  reads the per-line diff text directly.

P3-X3 — recent-restores line.
- hostChromeData grows RestoreStatus / RestoreAt / RestoreJobID
  populated via store.LatestJobByKind(host_id, 'restore') (already
  exists, used by the init line).
- host_chrome.html renders a small line below the existing init-status
  one with status-coloured copy + a link to the job log. Hidden when
  no restore has ever run on this host.

Tests:
- diff_test covers happy path (correct DiffPayload + HX-Redirect),
  same-id rejection (422), unknown-id rejection (422). Adds a
  seedTwoSnapshots helper since ReplaceHostSnapshots is atomic-swap
  (calling seedSnapshot twice would only leave the second).

Restage block (CLAUDE.md) deferred to the end of the restore phase.

internal/server/http/diff_test.go

@@ -0,0 +1,136 @@
+// diff_test.go — covers POST /api/hosts/{id}/snapshots/diff (P3-09).
+package http
+
+import (
+	"context"
+	"encoding/json"
+	stdhttp "net/http"
+	"net/url"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/coder/websocket"
+
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/api"
+)
+
+// TestSnapshotDiffHappyPath verifies a valid two-snapshot form ships
+// a JobDiff command.run with the right payload.
+func TestSnapshotDiffHappyPath(t *testing.T) {
+	t.Parallel()
+	srv, ts, st := rawTestServerWithUI(t)
+	hostID, token := enrolHostForUI(t, srv, st, "diff-host")
+	a, b := seedTwoSnapshots(t, st, hostID, "diff-host")
+	c := agentDial(t, srv, ts, hostID, token)
+	sendHello(t, c, "diff-host")
+	_ = drainUntil(t, c, api.MsgScheduleSet)
+	cookie := loginAsAdmin(t, st)
+
+	form := url.Values{
+		"snapshot_a": {a},
+		"snapshot_b": {b},
+	}
+	req, _ := stdhttp.NewRequest("POST",
+		ts.URL+"/hosts/"+hostID+"/snapshots/diff",
+		strings.NewReader(form.Encode()))
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	req.Header.Set("HX-Request", "true")
+	req.AddCookie(cookie)
+	client := &stdhttp.Client{
+		CheckRedirect: func(*stdhttp.Request, []*stdhttp.Request) error {
+			return stdhttp.ErrUseLastResponse
+		},
+	}
+	res, err := client.Do(req)
+	if err != nil {
+		t.Fatalf("do: %v", err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != stdhttp.StatusNoContent {
+		t.Fatalf("status: got %d, want 204", res.StatusCode)
+	}
+	if res.Header.Get("HX-Redirect") == "" {
+		t.Fatal("expected HX-Redirect to live job page")
+	}
+
+	deadline := time.Now().Add(2 * time.Second)
+	var got api.Envelope
+	for time.Now().Before(deadline) {
+		ctx, cancel := context.WithTimeout(context.Background(), 500*time.Millisecond)
+		mt, raw, rerr := c.Read(ctx)
+		cancel()
+		if rerr != nil {
+			break
+		}
+		if mt != websocket.MessageText {
+			continue
+		}
+		if !strings.Contains(string(raw), `"kind":"diff"`) {
+			continue
+		}
+		_ = json.Unmarshal(raw, &got)
+		break
+	}
+	if got.Type != api.MsgCommandRun {
+		t.Fatal("never received diff command.run")
+	}
+	var cp api.CommandRunPayload
+	_ = got.UnmarshalPayload(&cp)
+	if cp.Diff == nil {
+		t.Fatal("diff payload nil")
+	}
+	if cp.Diff.SnapshotA != a || cp.Diff.SnapshotB != b {
+		t.Fatalf("diff payload: got %+v want a=%s b=%s", cp.Diff, a, b)
+	}
+}
+
+// TestSnapshotDiffSameID rejects diff(a,a) with 422.
+func TestSnapshotDiffSameID(t *testing.T) {
+	t.Parallel()
+	srv, ts, st := rawTestServerWithUI(t)
+	hostID, _ := enrolHostForUI(t, srv, st, "diff-same")
+	a := seedSnapshot(t, st, hostID, "diff-same")
+	cookie := loginAsAdmin(t, st)
+
+	form := url.Values{"snapshot_a": {a}, "snapshot_b": {a}}
+	req, _ := stdhttp.NewRequest("POST",
+		ts.URL+"/hosts/"+hostID+"/snapshots/diff",
+		strings.NewReader(form.Encode()))
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	req.AddCookie(cookie)
+	res, err := stdhttp.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("do: %v", err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != stdhttp.StatusUnprocessableEntity {
+		t.Fatalf("status: got %d, want 422", res.StatusCode)
+	}
+	_ = srv
+}
+
+// TestSnapshotDiffUnknownID rejects ids not in the host's snapshot list.
+func TestSnapshotDiffUnknownID(t *testing.T) {
+	t.Parallel()
+	srv, ts, st := rawTestServerWithUI(t)
+	hostID, _ := enrolHostForUI(t, srv, st, "diff-unknown")
+	_ = seedSnapshot(t, st, hostID, "diff-unknown")
+	cookie := loginAsAdmin(t, st)
+
+	form := url.Values{"snapshot_a": {"deadbeef"}, "snapshot_b": {"cafebabe"}}
+	req, _ := stdhttp.NewRequest("POST",
+		ts.URL+"/hosts/"+hostID+"/snapshots/diff",
+		strings.NewReader(form.Encode()))
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	req.AddCookie(cookie)
+	res, err := stdhttp.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("do: %v", err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != stdhttp.StatusUnprocessableEntity {
+		t.Fatalf("status: got %d, want 422", res.StatusCode)
+	}
+	_ = srv
+}