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/config/config.go

@@ -41,7 +41,9 @@ type Config struct {
 }
 
 // New creates a new Config by reading from files and environment variables.
-func New(_ fx.Lifecycle, params Params) (*Config, error) {
+func New(
+	_ fx.Lifecycle, params Params,
+) (*Config, error) {
 	log := params.Logger.Get()
 	name := params.Globals.Appname
 
@@ -74,7 +76,7 @@ func New(_ fx.Lifecycle, params Params) (*Config, error) {
 		}
 	}
 
-	s := &Config{
+	cfg := &Config{
 		DBURL:           viper.GetString("DBURL"),
 		Debug:           viper.GetBool("DEBUG"),
 		Port:            viper.GetInt("PORT"),
@@ -92,10 +94,10 @@ func New(_ fx.Lifecycle, params Params) (*Config, error) {
 		params:          &params,
 	}
 
-	if s.Debug {
+	if cfg.Debug {
 		params.Logger.EnableDebugLogging()
-		s.log = params.Logger.Get()
+		cfg.log = params.Logger.Get()
 	}
 
-	return s, nil
+	return cfg, nil
 }