feat: implement unified directory structure with automatic migration for CalDAV and CardDAV

- Unify directory structure across protocols:
  * WebDAV: data/files/<username>/
  * CalDAV: data/<username>/calendars/<name>/
  * CardDAV: data/<username>/addressbooks/<name>/

- Add automatic migration capability that runs on server startup
- Maintain full backward compatibility with existing installations
- Improve Docker usage by automatically handling legacy data structure

- Updated storage provider implementations to use new nested structure
- Enhanced store functions for backward compatibility
- Modified CalDAV and CardDAV backends to use unified paths
- Added automatic migration logic in server initialization

internal/caldav/backend.go

@@ -116,7 +116,7 @@ func (b *Backend) ListCalendars(ctx context.Context) ([]caldav.Calendar, error)
 	disk, _ := b.store.ListCollections(p.Username)
 	configured := make(map[string]bool)
 	for _, cal := range names {
-		configured["cal-"+cal.Name] = true
+		configured["calendars/"+cal.Name] = true
 	}
 	for _, dir := range disk {
 		if strings.HasPrefix(dir, "cal-") && !configured[dir] {
@@ -188,7 +188,7 @@ func (b *Backend) GetCalendarObject(ctx context.Context, objPath string, req *ca
 		return nil, err
 	}
 
-	data, err := b.store.GetObject(owner, "cal-"+realName, objID)
+	data, err := b.store.GetObject(owner, "calendars/"+realName, objID)
 	if err != nil {
 		return nil, webdav.NewHTTPError(http.StatusNotFound, err)
 	}
@@ -212,7 +212,7 @@ func (b *Backend) ListCalendarObjects(ctx context.Context, calPath string, req *
 		return nil, err
 	}
 
-	ids, err := b.store.ListObjects(owner, "cal-"+realName)
+	ids, err := b.store.ListObjects(owner, "calendars/"+realName)
 	if err != nil {
 		return nil, err
 	}
@@ -260,7 +260,7 @@ func (b *Backend) CreateCalendar(ctx context.Context, calendar *caldav.Calendar)
 			return fmt.Errorf("registering calendar: %w", err)
 		}
 	}
-	return b.store.EnsureCollection(p.Username, "cal-"+name)
+	return b.store.EnsureCollection(p.Username, "calendars/"+name)
 }
 
 func (b *Backend) DeleteCalendar(ctx context.Context, calPath string) error {
@@ -285,7 +285,7 @@ func (b *Backend) DeleteCalendar(ctx context.Context, calPath string) error {
 	if err := b.dbase.DeleteCalendar(owner, realName); err != nil && err != db.ErrResourceNotFound {
 		return fmt.Errorf("unregistering calendar: %w", err)
 	}
-	return b.store.DeleteCollection(owner, "cal-"+realName)
+	return b.store.DeleteCollection(owner, "calendars/"+realName)
 }
 
 func (b *Backend) PutCalendarObject(ctx context.Context, objPath string, calendar *ical.Calendar, opts *caldav.PutCalendarObjectOptions) (*caldav.CalendarObject, error) {
@@ -311,7 +311,7 @@ func (b *Backend) PutCalendarObject(ctx context.Context, objPath string, calenda
 	}
 
 	data := []byte(buf.String())
-	if err := b.store.PutObject(owner, "cal-"+realName, objID, data); err != nil {
+	if err := b.store.PutObject(owner, "calendars/"+realName, objID, data); err != nil {
 		return nil, fmt.Errorf("storing calendar object: %w", err)
 	}
 
@@ -333,7 +333,7 @@ func (b *Backend) DeleteCalendarObject(ctx context.Context, objPath string) erro
 	if err != nil {
 		return err
 	}
-	if err := b.store.DeleteObject(owner, "cal-"+realName, objID); err != nil {
+	if err := b.store.DeleteObject(owner, "calendars/"+realName, objID); err != nil {
 		return webdav.NewHTTPError(http.StatusNotFound, err)
 	}
 	return nil

internal/caldav/backend_test.go

@@ -245,7 +245,7 @@ func TestPropFindExplicitCalendarColorRequest(t *testing.T) {
 	if err := dbase.CreateCalendarWithColor("alice", "work", "#3b82f6"); err != nil {
 		t.Fatalf("CreateCalendarWithColor: %v", err)
 	}
-	if err := st.EnsureCollection("alice", "cal-work"); err != nil {
+	if err := st.EnsureCollection("alice", "calendars/work"); err != nil {
 		t.Fatalf("EnsureCollection: %v", err)
 	}