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/storage.go

@@ -0,0 +1,24 @@
+// Package storage provides an abstraction layer for file content persistence.
+package storage
+
+import (
+	"context"
+	"io"
+)
+
+// StorageBackend abstracts where file content is written, read, and deleted.
+// Paths passed to all methods are relative to the backend's root and must
+// not contain path traversal sequences.
+type StorageBackend interface {
+	// Save writes the contents of reader to path, creating parent directories
+	// as needed. It returns the number of bytes written.
+	Save(ctx context.Context, path string, reader io.Reader) (int64, error)
+
+	// Open returns a reader for the file at path. The caller must close the
+	// returned ReadCloser.
+	Open(ctx context.Context, path string) (io.ReadCloser, error)
+
+	// Delete removes the file at path. It is idempotent — deleting a
+	// non-existent file is not an error.
+	Delete(ctx context.Context, path string) error
+}