feat: M4 gossip ops and UX

Refs #1

- /v1/diff endpoint returns each node's fingerprint set + tombstoned
  auto-thread status
- knox gossip diff <peer-url>: shows peer-only/local-only fingerprints
  (pull/keep preview) and tombstone divergence (resolved-on-peer vs
  would-resurrect)
- knox gossip sync: one-shot anti-entropy sweep + reconcile
- gossip server starts before initial seed so peers can reach a booting
  node; KNOX_PEER_ADDR alone now serves without KNOX_PEERS
- integration tests for diff + tombstone reporting
- e2e verified: two live daemons, diff previewed 1655 peer-only fps,
  sync converged second node to 1655 observations

internal/db/gossip.go

@@ -179,4 +179,44 @@ type Peer struct {
 	LastHandshake string
 	Cursor        int64
 	CreatedAt     string
+}
+
+// DistinctFingerprints returns the set of all observed fingerprints — the
+// ground-truth index of what this node knows. Used by gossip diff.
+func (k *KnoxDB) DistinctFingerprints() (map[string]bool, error) {
+	rows, err := k.db.Query(`SELECT DISTINCT fingerprint FROM observations`)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	set := make(map[string]bool)
+	for rows.Next() {
+		var f string
+		if err := rows.Scan(&f); err != nil {
+			return nil, err
+		}
+		set[f] = true
+	}
+	return set, rows.Err()
+}
+
+// ThreadStatusByCluster returns cluster_key → status for auto-threaded threads,
+// excluding human-created threads (no cluster key). Diff uses it to show
+// tombstoned threads: a thread resolved on one node but active on another.
+func (k *KnoxDB) ThreadStatusByCluster() (map[string]string, error) {
+	rows, err := k.db.Query(
+		`SELECT cluster_key, status FROM threads WHERE cluster_key IS NOT NULL AND cluster_key<>''`)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	out := make(map[string]string)
+	for rows.Next() {
+		var key, status string
+		if err := rows.Scan(&key, &status); err != nil {
+			return nil, err
+		}
+		out[key] = status
+	}
+	return out, rows.Err()
 }