Rig up CardDAV backend to HTTP server

References: https://todo.sr.ht/~sircmpwn/tokidoki/3

storage/postgresql.go

@@ -0,0 +1,38 @@
+package storage
+
+import (
+	"github.com/emersion/go-vcard"
+	"github.com/emersion/go-webdav/carddav"
+)
+
+type psqlBackend struct{}
+
+var _ carddav.Backend = (*psqlBackend)(nil)
+
+func NewPostgreSQL() carddav.Backend {
+	return &psqlBackend{}
+}
+
+func (*psqlBackend) AddressBook() (*carddav.AddressBook, error) {
+	panic("TODO")
+}
+
+func (*psqlBackend) GetAddressObject(path string, req *carddav.AddressDataRequest) (*carddav.AddressObject, error) {
+	panic("TODO")
+}
+
+func (*psqlBackend) ListAddressObjects(req *carddav.AddressDataRequest) ([]carddav.AddressObject, error) {
+	panic("TODO")
+}
+
+func (*psqlBackend) QueryAddressObjects(query *carddav.AddressBookQuery) ([]carddav.AddressObject, error) {
+	panic("TODO")
+}
+
+func (*psqlBackend) PutAddressObject(path string, card vcard.Card) (loc string, err error) {
+	panic("TODO")
+}
+
+func (*psqlBackend) DeleteAddressObject(path string) error {
+	panic("TODO")
+}