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/healthcheck/healthcheck.go

@@ -33,14 +33,17 @@ type Healthcheck struct {
 }
 
 // New creates a new Healthcheck instance.
-func New(lc fx.Lifecycle, params Params) (*Healthcheck, error) {
-	s := new(Healthcheck)
-	s.params = &params
-	s.log = params.Logger.Get()
+func New(
+	lifecycle fx.Lifecycle, params Params,
+) (*Healthcheck, error) {
+	hcheck := &Healthcheck{ //nolint:exhaustruct // StartupTime set in OnStart
+		params: &params,
+		log:    params.Logger.Get(),
+	}
 
-	lc.Append(fx.Hook{
+	lifecycle.Append(fx.Hook{
 		OnStart: func(_ context.Context) error {
-			s.StartupTime = time.Now()
+			hcheck.StartupTime = time.Now()
 
 			return nil
 		},
@@ -49,7 +52,7 @@ func New(lc fx.Lifecycle, params Params) (*Healthcheck, error) {
 		},
 	})
 
-	return s, nil
+	return hcheck, nil
 }
 
 // Response is the JSON response returned by the health endpoint.
@@ -64,19 +67,18 @@ type Response struct {
 }
 
 // Healthcheck returns the current health status of the server.
-func (s *Healthcheck) Healthcheck() *Response {
-	resp := &Response{
+func (hcheck *Healthcheck) Healthcheck() *Response {
+	return &Response{
 		Status:        "ok",
 		Now:           time.Now().UTC().Format(time.RFC3339Nano),
-		UptimeSeconds: int64(s.uptime().Seconds()),
-		UptimeHuman:   s.uptime().String(),
-		Appname:       s.params.Globals.Appname,
-		Version:       s.params.Globals.Version,
+		UptimeSeconds: int64(hcheck.uptime().Seconds()),
+		UptimeHuman:   hcheck.uptime().String(),
+		Appname:       hcheck.params.Globals.Appname,
+		Version:       hcheck.params.Globals.Version,
+		Maintenance:   hcheck.params.Config.MaintenanceMode,
 	}
-
-	return resp
 }
 
-func (s *Healthcheck) uptime() time.Duration {
-	return time.Since(s.StartupTime)
+func (hcheck *Healthcheck) uptime() time.Duration {
+	return time.Since(hcheck.StartupTime)
 }