fix: address all PR #10 review findings

Security:
- Add channel membership check before PRIVMSG (prevents non-members from sending)
- Add membership check on history endpoint (channels require membership, DMs scoped to own nick)
- Enforce MaxBytesReader on all POST request bodies
- Fix rand.Read error being silently ignored in token generation

Data integrity:
- Fix TOCTOU race in GetOrCreateChannel using INSERT OR IGNORE + SELECT

Build:
- Add CGO_ENABLED=0 to golangci-lint install in Dockerfile (fixes alpine build)

Linting:
- Strict .golangci.yml: only wsl disabled (deprecated in v2)
- Re-enable exhaustruct, depguard, godot, wrapcheck, varnamelen
- Fix linters-settings -> linters.settings for v2 config format
- Fix ALL lint findings in actual code (no linter config weakening)
- Wrap all external package errors (wrapcheck)
- Fill struct fields or add targeted nolint:exhaustruct where appropriate
- Rename short variables (ts->timestamp, n->bufIndex, etc.)
- Add depguard deny policy for io/ioutil and math/rand
- Exclude G704 (SSRF) in gosec config (CLI client takes user-configured URLs)

Tests:
- Add security tests (TestNonMemberCannotSend, TestHistoryNonMember)
- Split TestInsertAndPollMessages for reduced complexity
- Fix parallel test safety (viper global state prevents parallelism)
- Use t.Context() instead of context.Background() in tests

Docker build verified passing locally.

cmd/chat-cli/ui.go

@@ -32,10 +32,10 @@ type UI struct {
 
 // NewUI creates the tview-based IRC-like UI.
 func NewUI() *UI {
-	ui := &UI{
+	ui := &UI{ //nolint:exhaustruct,varnamelen // fields set below; ui is idiomatic
 		app: tview.NewApplication(),
 		buffers: []*Buffer{
-			{Name: "(status)", Lines: nil},
+			{Name: "(status)", Lines: nil, Unread: 0},
 		},
 	}
 
@@ -58,7 +58,12 @@ func NewUI() *UI {
 
 // Run starts the UI event loop (blocks).
 func (ui *UI) Run() error {
-	return ui.app.Run()
+	err := ui.app.Run()
+	if err != nil {
+		return fmt.Errorf("run ui: %w", err)
+	}
+
+	return nil
 }
 
 // Stop stops the UI.
@@ -100,15 +105,15 @@ func (ui *UI) AddStatus(line string) {
 }
 
 // SwitchBuffer switches to the buffer at index n.
-func (ui *UI) SwitchBuffer(n int) {
+func (ui *UI) SwitchBuffer(bufIndex int) {
 	ui.app.QueueUpdateDraw(func() {
-		if n < 0 || n >= len(ui.buffers) {
+		if bufIndex < 0 || bufIndex >= len(ui.buffers) {
 			return
 		}
 
-		ui.currentBuffer = n
+		ui.currentBuffer = bufIndex
 
-		buf := ui.buffers[n]
+		buf := ui.buffers[bufIndex]
 		buf.Unread = 0
 
 		ui.messages.Clear()
@@ -282,7 +287,7 @@ func (ui *UI) getOrCreateBuffer(name string) *Buffer {
 		}
 	}
 
-	buf := &Buffer{Name: name}
+	buf := &Buffer{Name: name, Lines: nil, Unread: 0}
 	ui.buffers = append(ui.buffers, buf)
 
 	return buf