fix(rpc): bind io_stream sessions to creator to prevent terminal/fm hijack

createTerminal and createFM correctly check server ownership before
issuing a stream UUID, but terminalStream and fmStream only verified
that the UUID existed. Any authenticated user holding a valid stream
UUID could attach to it, gaining the original creator's live shell or
file-manager session — and the UUID is exposed via URL path (referer
leaks, access logs, browser history, frontend error reporters).

Bind the creator user ID into ioStreamContext at CreateStream time,
expose StreamOwnership and IsStreamAuthorizedForUser, and check
ownership in terminalStream/fmStream before the WebSocket upgrade so a
rejected attempt does not tear down the legitimate stream via defer.

NAT streams are also routed through CreateStream(_, 0); they are not
reachable from /ws/terminal or /ws/file so a sentinel user ID is fine.

Co-authored-by: naiba/CloudCode <hi+cloudcode@nai.ba>

service/rpc/io_stream_test.go

@@ -12,7 +12,7 @@ func TestIOStream(t *testing.T) {
 
 	const testStreamID = "ffffffff-ffff-ffff-ffff-ffffffffffff"
 
-	handler.CreateStream(testStreamID)
+	handler.CreateStream(testStreamID, 0)
 	userIo, agentIo := newPipeReadWriter(), newPipeReadWriter()
 	defer func() {
 		userIo.Close()
@@ -105,3 +105,71 @@ func newPipeReadWriter() io.ReadWriteCloser {
 		io.WriteCloser
 	}{r, w}
 }
+
+func TestStreamOwnershipReturnsCreatorUserID(t *testing.T) {
+	h := NewNezhaHandler()
+	h.CreateStream("alice-stream", 100)
+
+	creator, found := h.StreamOwnership("alice-stream")
+	if !found {
+		t.Fatalf("expected stream to be found after CreateStream")
+	}
+	if creator != 100 {
+		t.Fatalf("expected creator user ID 100, got %d", creator)
+	}
+}
+
+func TestStreamOwnershipReturnsNotFoundForUnknownID(t *testing.T) {
+	h := NewNezhaHandler()
+	if _, found := h.StreamOwnership("nonexistent"); found {
+		t.Fatalf("expected unknown stream id to report not-found")
+	}
+}
+
+func TestStreamOwnershipPreservesPerStreamCreator(t *testing.T) {
+	h := NewNezhaHandler()
+	h.CreateStream("alice-stream", 100)
+	h.CreateStream("bob-stream", 200)
+
+	aliceCreator, _ := h.StreamOwnership("alice-stream")
+	bobCreator, _ := h.StreamOwnership("bob-stream")
+	if aliceCreator != 100 || bobCreator != 200 {
+		t.Fatalf("expected per-stream creator IDs alice=100 bob=200, got alice=%d bob=%d",
+			aliceCreator, bobCreator)
+	}
+}
+
+func TestIsStreamAuthorizedForUserAllowsCreator(t *testing.T) {
+	h := NewNezhaHandler()
+	h.CreateStream("alice-stream", 100)
+
+	if !h.IsStreamAuthorizedForUser("alice-stream", 100, false) {
+		t.Fatalf("creator must be authorized to attach to their own stream")
+	}
+}
+
+func TestIsStreamAuthorizedForUserDeniesForeignMember(t *testing.T) {
+	h := NewNezhaHandler()
+	h.CreateStream("alice-stream", 100)
+
+	if h.IsStreamAuthorizedForUser("alice-stream", 200, false) {
+		t.Fatalf("foreign member must not be authorized — session hijack would be possible")
+	}
+}
+
+func TestIsStreamAuthorizedForUserAllowsAdmin(t *testing.T) {
+	h := NewNezhaHandler()
+	h.CreateStream("alice-stream", 100)
+
+	if !h.IsStreamAuthorizedForUser("alice-stream", 999, true) {
+		t.Fatalf("admin must be authorized to attach regardless of creator")
+	}
+}
+
+func TestIsStreamAuthorizedForUserDeniesUnknownStream(t *testing.T) {
+	h := NewNezhaHandler()
+
+	if h.IsStreamAuthorizedForUser("nonexistent", 100, true) {
+		t.Fatalf("unknown stream id must not authorize even admin")
+	}
+}