Add file management API with local storage backend

- Implement FileHandler with CRUD operations for files/directories
- Add FileService with business logic and SHA-256 hashing
- Create LocalStorage backend for filesystem persistence
- Add database repository with pagination and name uniqueness
  constraints
- Configure max upload size in storage settings
- Include comprehensive tests for all layers

internal/storage/local.go

@@ -0,0 +1,91 @@
+package storage
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+)
+
+// LocalStorage implements StorageBackend using the local filesystem.
+type LocalStorage struct {
+	basePath string
+}
+
+// NewLocalStorage creates a LocalStorage rooted at basePath. The path is
+// resolved to an absolute path and verified to exist.
+func NewLocalStorage(basePath string) (*LocalStorage, error) {
+	abs, err := filepath.Abs(basePath)
+	if err != nil {
+		return nil, fmt.Errorf("resolve storage path: %w", err)
+	}
+	return &LocalStorage{basePath: abs}, nil
+}
+
+// Save writes the contents of reader to path under the storage root.
+func (s *LocalStorage) Save(_ context.Context, path string, reader io.Reader) (int64, error) {
+	fullPath := filepath.Join(s.basePath, path)
+	if !isSubPath(s.basePath, fullPath) {
+		return 0, fmt.Errorf("storage: path traversal attempt: %s", path)
+	}
+
+	if err := os.MkdirAll(filepath.Dir(fullPath), 0750); err != nil {
+		return 0, fmt.Errorf("create parent directory: %w", err)
+	}
+
+	file, err := os.Create(fullPath)
+	if err != nil {
+		return 0, fmt.Errorf("create file: %w", err)
+	}
+	defer file.Close()
+
+	written, err := io.Copy(file, reader)
+	if err != nil {
+		// Best-effort cleanup on write failure.
+		os.Remove(fullPath)
+		return 0, fmt.Errorf("write file: %w", err)
+	}
+
+	return written, nil
+}
+
+// Open returns a reader for the file at path under the storage root.
+func (s *LocalStorage) Open(_ context.Context, path string) (io.ReadCloser, error) {
+	fullPath := filepath.Join(s.basePath, path)
+	if !isSubPath(s.basePath, fullPath) {
+		return nil, fmt.Errorf("storage: path traversal attempt: %s", path)
+	}
+
+	file, err := os.Open(fullPath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, fmt.Errorf("file not found on disk: %s", path)
+		}
+		return nil, fmt.Errorf("open file: %w", err)
+	}
+	return file, nil
+}
+
+// Delete removes the file at path under the storage root.
+func (s *LocalStorage) Delete(_ context.Context, path string) error {
+	fullPath := filepath.Join(s.basePath, path)
+	if !isSubPath(s.basePath, fullPath) {
+		return fmt.Errorf("storage: path traversal attempt: %s", path)
+	}
+
+	err := os.Remove(fullPath)
+	if os.IsNotExist(err) {
+		return nil // idempotent: already gone
+	}
+	return err
+}
+
+// isSubPath returns true if target is within base (no path traversal).
+func isSubPath(base, target string) bool {
+	rel, err := filepath.Rel(base, target)
+	if err != nil {
+		return false
+	}
+	return rel != ".." && !filepath.IsAbs(rel) && rel[:2] != ".."
+}