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/healthcheck.go

@@ -7,9 +7,12 @@ import (
 const httpStatusOK = 200
 
 // HandleHealthCheck returns an HTTP handler for the health check endpoint.
-func (s *Handlers) HandleHealthCheck() http.HandlerFunc {
-	return func(w http.ResponseWriter, req *http.Request) {
-		resp := s.hc.Healthcheck()
-		s.respondJSON(w, req, resp, httpStatusOK)
+func (hdlr *Handlers) HandleHealthCheck() http.HandlerFunc {
+	return func(
+		writer http.ResponseWriter,
+		request *http.Request,
+	) {
+		resp := hdlr.hc.Healthcheck()
+		hdlr.respondJSON(writer, request, resp, httpStatusOK)
 	}
 }