Fix SEC-08: make system token issuance atomic

- Add IssueSystemToken() method in internal/db/accounts.go that wraps
  revoke-old, track-new, and upsert-system_tokens in a single SQLite
  transaction
- Update handleTokenIssue in internal/server/server.go to use the new
  atomic method instead of three separate DB calls
- Update IssueServiceToken in internal/grpcserver/tokenservice.go with
  the same fix
- Add TestIssueSystemTokenAtomic test covering first issue and rotation

Security: token issuance now uses a single transaction to prevent
inconsistent state (e.g., old token revoked but new token not tracked)
if a crash occurs between operations. Follows the same pattern as
RenewToken which was already correctly transactional.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/db/db_test.go

@@ -445,6 +445,79 @@ func TestSystemTokenRotationRevokesOld(t *testing.T) {
 	}
 }
 
+// TestIssueSystemTokenAtomic verifies that IssueSystemToken atomically
+// revokes an old token, tracks the new token, and upserts system_tokens.
+func TestIssueSystemTokenAtomic(t *testing.T) {
+	db := openTestDB(t)
+	acct, err := db.CreateAccount("svc-atomic", model.AccountTypeSystem, "hash")
+	if err != nil {
+		t.Fatalf("CreateAccount: %v", err)
+	}
+
+	now := time.Now().UTC()
+	exp := now.Add(time.Hour)
+
+	// Issue first system token with no old JTI.
+	jti1 := "atomic-sys-tok-1"
+	if err := db.IssueSystemToken("", jti1, acct.ID, now, exp); err != nil {
+		t.Fatalf("IssueSystemToken first: %v", err)
+	}
+
+	// Verify the first token is tracked and not revoked.
+	rec1, err := db.GetTokenRecord(jti1)
+	if err != nil {
+		t.Fatalf("GetTokenRecord jti1: %v", err)
+	}
+	if rec1.IsRevoked() {
+		t.Error("first token should not be revoked")
+	}
+
+	// Verify system_tokens points to the first token.
+	st1, err := db.GetSystemToken(acct.ID)
+	if err != nil {
+		t.Fatalf("GetSystemToken after first issue: %v", err)
+	}
+	if st1.JTI != jti1 {
+		t.Errorf("system token JTI = %q, want %q", st1.JTI, jti1)
+	}
+
+	// Issue second token, which should atomically revoke the first.
+	jti2 := "atomic-sys-tok-2"
+	if err := db.IssueSystemToken(jti1, jti2, acct.ID, now, exp); err != nil {
+		t.Fatalf("IssueSystemToken second: %v", err)
+	}
+
+	// First token must be revoked.
+	rec1After, err := db.GetTokenRecord(jti1)
+	if err != nil {
+		t.Fatalf("GetTokenRecord jti1 after rotation: %v", err)
+	}
+	if !rec1After.IsRevoked() {
+		t.Error("first token should be revoked after second issue")
+	}
+	if rec1After.RevokeReason != "rotated" {
+		t.Errorf("revoke reason = %q, want %q", rec1After.RevokeReason, "rotated")
+	}
+
+	// Second token must be tracked and not revoked.
+	rec2, err := db.GetTokenRecord(jti2)
+	if err != nil {
+		t.Fatalf("GetTokenRecord jti2: %v", err)
+	}
+	if rec2.IsRevoked() {
+		t.Error("second token should not be revoked")
+	}
+
+	// system_tokens must point to the second token.
+	st2, err := db.GetSystemToken(acct.ID)
+	if err != nil {
+		t.Fatalf("GetSystemToken after second issue: %v", err)
+	}
+	if st2.JTI != jti2 {
+		t.Errorf("system token JTI = %q, want %q", st2.JTI, jti2)
+	}
+}
+
 func TestRevokeAllUserTokens(t *testing.T) {
 	db := openTestDB(t)
 	acct, err := db.CreateAccount("ivan", model.AccountTypeHuman, "hash")