security: enforce channel membership check in handleTopic (#75)

## Summary

`handleTopic` in `internal/handlers/api.go` did NOT check that the user was a member of the channel before allowing them to set a topic. Any authenticated user could set the topic on any channel they hadn't joined.

## Changes

- **`internal/handlers/api.go`**: Added `IsChannelMember` check after resolving the channel ID and before calling `executeTopic`, mirroring the existing pattern in `handleChannelMsg`. Non-members now receive `ERR_NOTONCHANNEL` (442).
- **`internal/handlers/api_test.go`**: Added `TestTopicNonMember` — creates a channel with one user, then verifies a second user who hasn't joined receives numeric 442 when attempting to set the topic.

## Testing

- All existing tests pass
- New `TestTopicNonMember` test validates the fix
- `docker build .` passes clean (formatting, linting, tests, build)

closes #33

Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: #75
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/handlers/api.go

@@ -1641,6 +1641,32 @@ func (hdlr *Handlers) handleTopic(
 		return
 	}
 
+	isMember, err := hdlr.params.Database.IsChannelMember(
+		request.Context(), chID, sessionID,
+	)
+	if err != nil {
+		hdlr.log.Error(
+			"check membership failed", "error", err,
+		)
+		hdlr.respondError(
+			writer, request,
+			"internal error",
+			http.StatusInternalServerError,
+		)
+
+		return
+	}
+
+	if !isMember {
+		hdlr.respondIRCError(
+			writer, request, clientID, sessionID,
+			irc.ErrNotOnChannel, nick, []string{channel},
+			"You're not on that channel",
+		)
+
+		return
+	}
+
 	hdlr.executeTopic(
 		writer, request,
 		sessionID, clientID, nick,

internal/handlers/api_test.go

@@ -1140,6 +1140,42 @@ func TestTopicMissingBody(t *testing.T) {
 	}
 }
 
+func TestTopicNonMember(t *testing.T) {
+	tserver := newTestServer(t)
+	aliceToken := tserver.createSession("alice_topic")
+	bobToken := tserver.createSession("bob_topic")
+
+	// Only alice joins the channel.
+	tserver.sendCommand(aliceToken, map[string]any{
+		commandKey: joinCmd, toKey: "#topicpriv",
+	})
+
+	// Drain bob's initial messages.
+	_, lastID := tserver.pollMessages(bobToken, 0)
+
+	// Bob tries to set topic without joining.
+	status, _ := tserver.sendCommand(
+		bobToken,
+		map[string]any{
+			commandKey: "TOPIC",
+			toKey:      "#topicpriv",
+			bodyKey:    []string{"Hijacked topic"},
+		},
+	)
+	if status != http.StatusOK {
+		t.Fatalf("expected 200, got %d", status)
+	}
+
+	msgs, _ := tserver.pollMessages(bobToken, lastID)
+
+	if !findNumeric(msgs, "442") {
+		t.Fatalf(
+			"expected ERR_NOTONCHANNEL (442), got %v",
+			msgs,
+		)
+	}
+}
+
 func TestPing(t *testing.T) {
 	tserver := newTestServer(t)
 	token := tserver.createSession("ping_user")