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.

Dockerfile

@@ -4,7 +4,7 @@ WORKDIR /src
 RUN apk add --no-cache git build-base make
 
 # golangci-lint v2.1.6 (eabc2638a66d), 2026-02-26
-RUN go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@eabc2638a66daf5bb6c6fb052a32fa3ef7b6600d
+RUN CGO_ENABLED=0 go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@eabc2638a66daf5bb6c6fb052a32fa3ef7b6600d
 
 COPY go.mod go.sum ./
 RUN go mod download