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/db/export_test.go

@@ -13,35 +13,48 @@ var testDBCounter atomic.Int64
 
 // NewTestDatabase creates an in-memory database for testing.
 func NewTestDatabase() (*Database, error) {
-	n := testDBCounter.Add(1)
+	counter := testDBCounter.Add(1)
 
 	dsn := fmt.Sprintf(
 		"file:testdb%d?mode=memory"+
 			"&cache=shared&_pragma=foreign_keys(1)",
-		n,
+		counter,
 	)
 
-	d, err := sql.Open("sqlite", dsn)
+	conn, err := sql.Open("sqlite", dsn)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("open test db: %w", err)
 	}
 
-	database := &Database{db: d, log: slog.Default()}
+	database := &Database{ //nolint:exhaustruct // test helper, params not needed
+		conn: conn,
+		log:  slog.Default(),
+	}
 
 	err = database.runMigrations(context.Background())
 	if err != nil {
-		closeErr := d.Close()
+		closeErr := conn.Close()
 		if closeErr != nil {
-			return nil, closeErr
+			return nil, fmt.Errorf(
+				"close after migration failure: %w",
+				closeErr,
+			)
 		}
 
-		return nil, err
+		return nil, fmt.Errorf(
+			"run test migrations: %w", err,
+		)
 	}
 
 	return database, nil
 }
 
 // Close closes the underlying database connection.
-func (s *Database) Close() error {
-	return s.db.Close()
+func (database *Database) Close() error {
+	err := database.conn.Close()
+	if err != nil {
+		return fmt.Errorf("close database: %w", err)
+	}
+
+	return nil
 }