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.go

@@ -0,0 +1,150 @@
+package http
+
+import (
+	"encoding/json"
+	stdhttp "net/http"
+	"strings"
+	"time"
+
+	"github.com/go-chi/chi/v5"
+	"github.com/oklog/ulid/v2"
+
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/api"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/store"
+)
+
+// snapshotDiffRequest is the JSON body for POST .../snapshots/diff.
+// Either short or long snapshot IDs are accepted (restic's diff
+// command takes both).
+type snapshotDiffRequest struct {
+	SnapshotA string `json:"snapshot_a"`
+	SnapshotB string `json:"snapshot_b"`
+}
+
+// handleSnapshotDiff dispatches a JobDiff. Output streams as
+// log.stream lines to the standard live job page; the operator reads
+// the diff text directly there. Behaves like the run-now endpoints:
+// 503 if the host is offline, 400 if the IDs are missing, 422 if
+// they're not in the host's snapshot list (we don't want operators
+// running diffs against arbitrary snapshot strings).
+func (s *Server) handleSnapshotDiff(w stdhttp.ResponseWriter, r *stdhttp.Request) {
+	user, ok := s.requireUser(r)
+	if !ok {
+		writeJSONError(w, stdhttp.StatusUnauthorized, "unauthorized", "")
+		return
+	}
+	hostID := chi.URLParam(r, "id")
+	host, err := s.deps.Store.GetHost(r.Context(), hostID)
+	if err != nil {
+		writeJSONError(w, stdhttp.StatusNotFound, "host_not_found", "")
+		return
+	}
+
+	var req snapshotDiffRequest
+	// HTMX form posts arrive as application/x-www-form-urlencoded;
+	// the JSON shape is also accepted for REST callers.
+	ct := r.Header.Get("Content-Type")
+	if strings.HasPrefix(ct, "application/x-www-form-urlencoded") {
+		if err := r.ParseForm(); err != nil {
+			writeJSONError(w, stdhttp.StatusBadRequest, "invalid_form", err.Error())
+			return
+		}
+		req.SnapshotA = strings.TrimSpace(r.PostForm.Get("snapshot_a"))
+		req.SnapshotB = strings.TrimSpace(r.PostForm.Get("snapshot_b"))
+	} else {
+		if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+			writeJSONError(w, stdhttp.StatusBadRequest, "invalid_json", err.Error())
+			return
+		}
+		req.SnapshotA = strings.TrimSpace(req.SnapshotA)
+		req.SnapshotB = strings.TrimSpace(req.SnapshotB)
+	}
+	if req.SnapshotA == "" || req.SnapshotB == "" {
+		writeJSONError(w, stdhttp.StatusBadRequest, "missing_snapshot",
+			"snapshot_a and snapshot_b are both required")
+		return
+	}
+	if req.SnapshotA == req.SnapshotB {
+		writeJSONError(w, stdhttp.StatusUnprocessableEntity, "same_snapshot",
+			"diff requires two different snapshots")
+		return
+	}
+
+	// Validate the IDs are known to this host. Match on long ID, short
+	// ID, or any prefix match — operators sometimes paste a 6-char
+	// shortened form.
+	snaps, err := s.deps.Store.ListSnapshotsByHost(r.Context(), host.ID)
+	if err != nil {
+		writeJSONError(w, stdhttp.StatusInternalServerError, "internal", "")
+		return
+	}
+	resolveID := func(idOrShort string) string {
+		for _, s := range snaps {
+			if s.ID == idOrShort || s.ShortID == idOrShort {
+				return s.ID
+			}
+		}
+		// Prefix fallback (operator pasted 6 chars of a long id).
+		for _, s := range snaps {
+			if strings.HasPrefix(s.ID, idOrShort) {
+				return s.ID
+			}
+		}
+		return ""
+	}
+	a := resolveID(req.SnapshotA)
+	b := resolveID(req.SnapshotB)
+	if a == "" || b == "" {
+		writeJSONError(w, stdhttp.StatusUnprocessableEntity, "snapshot_not_found",
+			"one or both snapshot ids are not in this host's snapshot list")
+		return
+	}
+
+	if !s.deps.Hub.Connected(host.ID) {
+		writeJSONError(w, stdhttp.StatusServiceUnavailable, "host_offline",
+			"agent is not connected; try again when it reconnects")
+		return
+	}
+
+	jobID := ulid.Make().String()
+	now := time.Now().UTC()
+	if err := s.deps.Store.CreateJob(r.Context(), store.Job{
+		ID: jobID, HostID: host.ID, Kind: string(api.JobDiff),
+		ActorKind: "user", ActorID: &user.ID, CreatedAt: now,
+	}); err != nil {
+		writeJSONError(w, stdhttp.StatusInternalServerError, "internal", err.Error())
+		return
+	}
+	env, err := api.Marshal(api.MsgCommandRun, jobID, api.CommandRunPayload{
+		JobID: jobID, Kind: api.JobDiff,
+		Diff: &api.DiffPayload{SnapshotA: a, SnapshotB: b},
+	})
+	if err != nil {
+		writeJSONError(w, stdhttp.StatusInternalServerError, "internal", "")
+		return
+	}
+	if err := s.deps.Hub.Send(r.Context(), host.ID, env); err != nil {
+		writeJSONError(w, stdhttp.StatusServiceUnavailable, "host_offline", err.Error())
+		return
+	}
+	_ = s.deps.Store.AppendAudit(r.Context(), store.AuditEntry{
+		ID:         ulid.Make().String(),
+		UserID:     &user.ID,
+		Actor:      "user",
+		Action:     "host.snapshot_diff",
+		TargetKind: ptr("host"),
+		TargetID:   &host.ID,
+		TS:         now,
+	})
+
+	jobURL := "/jobs/" + jobID
+	if r.Header.Get("HX-Request") == "true" {
+		w.Header().Set("HX-Redirect", jobURL)
+		w.WriteHeader(stdhttp.StatusNoContent)
+		return
+	}
+	writeJSON(w, stdhttp.StatusAccepted, map[string]string{
+		"job_id":  jobID,
+		"job_url": jobURL,
+	})
+}