branding: make product identity runtime-configurable

Adopt upstream owpengram/owpengram-server's branding.Config/Configure/
Current in place of the old package-level constants. This is the piece the
earlier merge attempt was blocked on (internal/branding failed to import
during that merge). The default identity is unchanged -- every existing
ProductName/ProductUsername/... default still reads "OwpenGram" -- so this
is a pure capability add: nothing currently calls Configure, and every call
site now reads the current snapshot via a function instead of a compile-time
constant.

Five string templates that concatenated branding.ProductName into a `const`
had to become `var`, since a func call is no longer a valid const operand.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/app/botverification/seed.go

@@ -42,7 +42,7 @@ func (s *Service) SeedDefaultVerifier(ctx context.Context) (bool, error) {
 	if _, err := s.GrantVerifier(ctx, domain.BotVerifierSettings{
 		BotID:                      domain.VerifierBotUserID,
 		IconDocumentID:             icon.DocumentID,
-		CompanyName:                branding.ProductName,
+		CompanyName:                branding.ProductName(),
 		DefaultDescription:         "Bundled reference verifier -- auto-granted on first boot.",
 		CanModifyCustomDescription: false,
 		Enabled:                    true,