style: adopt golangci-lint v2 and resolve all findings

Add .golangci.yml (v2): the standard linters plus bodyclose, errorlint, noctx, misspell, rowserrcheck, sqlclosecheck, unconvert, usestdlibvars, with gofumpt + goimports formatters. Enable the std-error-handling exclusion preset for idiomatic Close/Remove/Setenv ignores; scope-exclude SA1019 (parser.ParseDir in tools/openapigen) and ST1005 (intentional capitalized user-facing error copy that tests assert verbatim). No inline nolint directives were introduced.

Resolve all 217 findings behavior-preserving: gofumpt/goimports formatting, explicit blank assignment on intentionally ignored errors, errors.Is/errors.As and %w wrapping, context-aware stdlib calls (CommandContext/QueryContext/NewRequestWithContext/Dialer), staticcheck simplifications, removed redundant conversions, http.StatusOK and http.MethodGet, inlined the go:fix intPtr helper, and deferred sql rows Close. Add a golangci CI job mirroring the existing Go jobs.

internal/sub/external_subscription_test.go

@@ -1,6 +1,7 @@
 package sub
 
 import (
+	"errors"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -14,7 +15,7 @@ func TestDoFetchSubscriptionLinks_RejectsOversizedBody(t *testing.T) {
 	defer srv.Close()
 
 	links, err := doFetchSubscriptionLinks(srv.URL)
-	if err != errSubscriptionBodyTooLarge {
+	if !errors.Is(err, errSubscriptionBodyTooLarge) {
 		t.Fatalf("err = %v, want errSubscriptionBodyTooLarge", err)
 	}
 	if links != nil {