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.

internal/handlers/handlers.go

@@ -40,40 +40,59 @@ type Handlers struct {
 
 // New creates a new Handlers instance.
 func New(
-	lc fx.Lifecycle,
+	lifecycle fx.Lifecycle,
 	params Params,
 ) (*Handlers, error) {
-	s := new(Handlers)
-	s.params = &params
-	s.log = params.Logger.Get()
-	s.hc = params.Healthcheck
-	s.broker = broker.New()
+	hdlr := &Handlers{
+		params: &params,
+		log:    params.Logger.Get(),
+		hc:     params.Healthcheck,
+		broker: broker.New(),
+	}
 
-	lc.Append(fx.Hook{
+	lifecycle.Append(fx.Hook{
 		OnStart: func(_ context.Context) error {
 			return nil
 		},
+		OnStop: func(_ context.Context) error {
+			return nil
+		},
 	})
 
-	return s, nil
+	return hdlr, nil
 }
 
-func (s *Handlers) respondJSON(
-	w http.ResponseWriter,
+func (hdlr *Handlers) respondJSON(
+	writer http.ResponseWriter,
 	_ *http.Request,
 	data any,
 	status int,
 ) {
-	w.Header().Set(
+	writer.Header().Set(
 		"Content-Type",
 		"application/json; charset=utf-8",
 	)
-	w.WriteHeader(status)
+	writer.WriteHeader(status)
 
 	if data != nil {
-		err := json.NewEncoder(w).Encode(data)
+		err := json.NewEncoder(writer).Encode(data)
 		if err != nil {
-			s.log.Error("json encode error", "error", err)
+			hdlr.log.Error(
+				"json encode error", "error", err,
+			)
 		}
 	}
 }
+
+func (hdlr *Handlers) respondError(
+	writer http.ResponseWriter,
+	request *http.Request,
+	msg string,
+	status int,
+) {
+	hdlr.respondJSON(
+		writer, request,
+		map[string]string{"error": msg},
+		status,
+	)
+}