Enforce Go style rules across codebase

Apply five style rules: convert iota string enums to typed
string constants, replace errors.As with errors.AsType,
merge three-group imports into two groups, fix multiline
parameter/argument formatting, and replace fmt.Sprintf URL
construction with net/url.

Signed-off-by: Émile Ré <emile@probo.com>

pkg/coredata/document_version_signature.go

@@ -216,8 +216,7 @@ INSERT INTO document_version_signatures (
 
 	_, err := conn.Exec(ctx, q, args)
 	if err != nil {
-		var pgErr *pgconn.PgError
-		if errors.As(err, &pgErr) {
+		if pgErr, ok := errors.AsType[*pgconn.PgError](err); ok {
 			if pgErr.Code == "23505" && pgErr.ConstraintName == "policy_version_signatures_policy_version_id_signed_by_key" {
 				return ErrResourceAlreadyExists
 			}