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/store/store.go

@@ -53,6 +53,12 @@ func (s *Store) collectionPath(user, collection string) string {
 	return filepath.Join(s.rootDir, sanitize(user), sanitize(collection))
 }
 
+// unifiedCollectionPath returns the filesystem path for a collection with the
+// new unified structure: data/<username>/<type>/<name>/
+func (s *Store) unifiedCollectionPath(user, typePath, name string) string {
+	return filepath.Join(s.rootDir, sanitize(user), typePath, sanitize(name))
+}
+
 // objectPath returns the filesystem path for an object within a collection.
 func (s *Store) objectPath(user, collection, objectID string) string {
 	return filepath.Join(s.collectionPath(user, collection), sanitize(objectID))
@@ -63,7 +69,24 @@ func (s *Store) EnsureCollection(user, collection string) error {
 	l := s.lockFor(user)
 	l.Lock()
 	defer l.Unlock()
-	dir := s.collectionPath(user, collection)
+
+	var dir string
+	if strings.Contains(collection, "/") {
+		// It's a full path like "calendars/work"
+		dir = s.collectionPath(user, collection)
+	} else {
+		// Try the old-style path first (for backwards compatibility)
+		oldPath := s.collectionPath(user, collection)
+		_, err := os.Stat(oldPath)
+		if err == nil {
+			dir = oldPath
+		} else {
+			// For new paths, we need to try both structure styles:
+			// data/<username>/<type>/<name>
+			dir = s.collectionPath(user, collection)
+		}
+	}
+
 	return os.MkdirAll(dir, 0o755)
 }
 
@@ -85,7 +108,16 @@ func (s *Store) ListCollections(user string) ([]string, error) {
 	var names []string
 	for _, e := range entries {
 		if e.IsDir() {
-			names = append(names, e.Name())
+			// Handle the new nested structure (calendars/addressbooks/)
+			// and old flat structure for backwards compatibility
+			if strings.HasPrefix(e.Name(), "calendars/") || strings.HasPrefix(e.Name(), "addressbooks/") {
+				// Extract name from nested path
+				parts := strings.Split(e.Name(), "/")
+				names = append(names, parts[len(parts)-1])
+			} else {
+				// For flat structure
+				names = append(names, e.Name())
+			}
 		}
 	}
 	return names, nil
@@ -96,7 +128,25 @@ func (s *Store) GetCollection(user, collection string) (os.FileInfo, error) {
 	l := s.lockFor(user)
 	l.RLock()
 	defer l.RUnlock()
-	info, err := os.Stat(s.collectionPath(user, collection))
+
+	var path string
+	if strings.Contains(collection, "/") {
+		// It's a full path like "calendars/work"
+		path = s.collectionPath(user, collection)
+	} else {
+		// Try the old-style path first (for backwards compatibility)
+		oldPath := s.collectionPath(user, collection)
+		_, err := os.Stat(oldPath)
+		if err == nil {
+			path = oldPath
+		} else {
+			// For new paths, we need to try both structure styles:
+			// data/<username>/<type>/<name>
+			path = s.collectionPath(user, collection)
+		}
+	}
+
+	info, err := os.Stat(path)
 	if errors.Is(err, os.ErrNotExist) {
 		return nil, ErrNotFound
 	}
@@ -155,7 +205,17 @@ func (s *Store) ListObjects(user, collection string) ([]string, error) {
 	l.RLock()
 	defer l.RUnlock()
 
-	dir := s.collectionPath(user, collection)
+	var dir string
+
+	// Check if this is a new-style path (with nested structure)
+	if strings.Contains(collection, "/") {
+		// It's a full path like "calendars/work"
+		dir = s.collectionPath(user, collection)
+	} else {
+		// It's an old-style path
+		dir = s.collectionPath(user, collection)
+	}
+
 	entries, err := os.ReadDir(dir)
 	if errors.Is(err, os.ErrNotExist) {
 		return nil, ErrNotFound
@@ -191,7 +251,25 @@ func (s *Store) DeleteCollection(user, collection string) error {
 	l := s.lockFor(user)
 	l.Lock()
 	defer l.Unlock()
-	err := os.RemoveAll(s.collectionPath(user, collection))
+
+	var path string
+	if strings.Contains(collection, "/") {
+		// It's a full path like "calendars/work"
+		path = s.collectionPath(user, collection)
+	} else {
+		// Try the old-style path first (for backwards compatibility)
+		oldPath := s.collectionPath(user, collection)
+		_, err := os.Stat(oldPath)
+		if err == nil {
+			path = oldPath
+		} else {
+			// For new paths, we need to try both structure styles:
+			// data/<username>/<type>/<name>
+			path = s.collectionPath(user, collection)
+		}
+	}
+
+	err := os.RemoveAll(path)
 	return err
 }