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.go

@@ -1,6 +1,7 @@
 package sub
 
 import (
+	"context"
 	"encoding/base64"
 	"io"
 	"net/http"
@@ -64,7 +65,7 @@ func fetchSubscriptionLinks(rawURL string) []string {
 }
 
 func doFetchSubscriptionLinks(rawURL string) ([]string, error) {
-	req, err := http.NewRequest(http.MethodGet, rawURL, nil)
+	req, err := http.NewRequestWithContext(context.Background(), http.MethodGet, rawURL, nil)
 	if err != nil {
 		return nil, err
 	}