Fix F-16: revoke old system token before issuing new one

- ui/handlers_accounts.go (handleIssueSystemToken): call
  GetSystemToken before issuing; if one exists, call
  RevokeToken(existing.JTI, "rotated") before TrackToken
  and SetSystemToken for the new token; mirrors the pattern
  in REST handleTokenIssue and gRPC IssueServiceToken
- db/db_test.go: TestSystemTokenRotationRevokesOld verifies
  the full rotation flow: old JTI revoked with reason
  "rotated", new JTI tracked and active, GetSystemToken
  returns the new JTI
- AUDIT.md: mark F-16 as fixed
Security: without this fix an old system token remained valid
  after rotation until its natural expiry, giving a leaked or
  stolen old token extra lifetime. With the revocation the old
  JTI is immediately marked in token_revocation so any validator
  checking revocation status rejects it.

internal/db/db_test.go

@@ -375,6 +375,76 @@ func TestRenewTokenAtomic(t *testing.T) {
 	}
 }
 
+// TestSystemTokenRotationRevokesOld verifies the rotation pattern used by
+// handleIssueSystemToken (F-16): issuing a second system token revokes the first.
+func TestSystemTokenRotationRevokesOld(t *testing.T) {
+	db := openTestDB(t)
+	acct, err := db.CreateAccount("svc", model.AccountTypeSystem, "hash")
+	if err != nil {
+		t.Fatalf("CreateAccount: %v", err)
+	}
+
+	now := time.Now().UTC()
+	exp := now.Add(time.Hour)
+
+	// Issue first token.
+	jti1 := "sys-tok-1"
+	if err := db.TrackToken(jti1, acct.ID, now, exp); err != nil {
+		t.Fatalf("TrackToken jti1: %v", err)
+	}
+	if err := db.SetSystemToken(acct.ID, jti1, exp); err != nil {
+		t.Fatalf("SetSystemToken jti1: %v", err)
+	}
+
+	// Simulate token rotation: look up existing, revoke it, issue second.
+	existing, err := db.GetSystemToken(acct.ID)
+	if err != nil {
+		t.Fatalf("GetSystemToken: %v", err)
+	}
+	if existing.JTI != jti1 {
+		t.Errorf("expected JTI %q, got %q", jti1, existing.JTI)
+	}
+	_ = db.RevokeToken(existing.JTI, "rotated")
+
+	jti2 := "sys-tok-2"
+	if err := db.TrackToken(jti2, acct.ID, now, exp); err != nil {
+		t.Fatalf("TrackToken jti2: %v", err)
+	}
+	if err := db.SetSystemToken(acct.ID, jti2, exp); err != nil {
+		t.Fatalf("SetSystemToken jti2: %v", err)
+	}
+
+	// Old token must be revoked.
+	old, err := db.GetTokenRecord(jti1)
+	if err != nil {
+		t.Fatalf("GetTokenRecord jti1: %v", err)
+	}
+	if !old.IsRevoked() {
+		t.Error("old system token should be revoked after rotation")
+	}
+	if old.RevokeReason != "rotated" {
+		t.Errorf("revoke reason = %q, want %q", old.RevokeReason, "rotated")
+	}
+
+	// New token must be active.
+	newRec, err := db.GetTokenRecord(jti2)
+	if err != nil {
+		t.Fatalf("GetTokenRecord jti2: %v", err)
+	}
+	if newRec.IsRevoked() {
+		t.Error("new system token should not be revoked after rotation")
+	}
+
+	// GetSystemToken must return the new JTI.
+	cur, err := db.GetSystemToken(acct.ID)
+	if err != nil {
+		t.Fatalf("GetSystemToken after rotation: %v", err)
+	}
+	if cur.JTI != jti2 {
+		t.Errorf("current system token JTI = %q, want %q", cur.JTI, jti2)
+	}
+}
+
 func TestRevokeAllUserTokens(t *testing.T) {
 	db := openTestDB(t)
 	acct, err := db.CreateAccount("ivan", model.AccountTypeHuman, "hash")