feat: implement unified S3-compatible storage system

Consolidates storage backends into a single S3-compatible driver that supports:
- AWS S3 (native)
- Backblaze B2 (S3-compatible API)
- Cloudflare R2 (S3-compatible API)
- MinIO and other S3-compatible services
- Local filesystem for development

This replaces the previous separate B2 driver with a unified approach,
reducing dependencies and complexity while adding support for more services.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

core/storage/interface_test.go

@@ -0,0 +1,101 @@
+package storage
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNewStorage(t *testing.T) {
+	t.Run("Local Storage", func(t *testing.T) {
+		config := Config{
+			Backend: "local",
+			Local: LocalConfig{
+				RootDir: t.TempDir(),
+			},
+		}
+		
+		storage, err := NewStorage(config)
+		assert.NoError(t, err)
+		assert.IsType(t, &LocalStorage{}, storage)
+	})
+	
+	t.Run("Default to Local Storage", func(t *testing.T) {
+		config := Config{
+			// No backend specified
+			Local: LocalConfig{
+				RootDir: t.TempDir(),
+			},
+		}
+		
+		storage, err := NewStorage(config)
+		assert.NoError(t, err)
+		assert.IsType(t, &LocalStorage{}, storage)
+	})
+	
+	t.Run("S3 Storage", func(t *testing.T) {
+		config := Config{
+			Backend: "s3",
+			S3: S3Config{
+				Region: "us-east-1",
+				Bucket: "test-bucket",
+			},
+		}
+		
+		// This might succeed if AWS credentials are available via IAM roles or env vars
+		// Let's just check that we get an S3Storage instance or an error
+		storage, err := NewStorage(config)
+		if err != nil {
+			// If it fails, that's expected in test environments without AWS access
+			assert.Nil(t, storage)
+		} else {
+			// If it succeeds, we should get an S3Storage instance
+			assert.IsType(t, &S3Storage{}, storage)
+		}
+	})
+	
+	t.Run("B2 Storage", func(t *testing.T) {
+		config := Config{
+			Backend: "b2",
+			B2: B2Config{
+				AccountID:      "test-account",
+				ApplicationKey: "test-key",
+				Bucket:         "test-bucket",
+			},
+		}
+		
+		// This will fail because we don't have real B2 credentials
+		storage, err := NewStorage(config)
+		assert.Error(t, err) // Expected to fail without credentials
+		assert.Nil(t, storage)
+	})
+	
+	t.Run("Unsupported Backend", func(t *testing.T) {
+		config := Config{
+			Backend: "unsupported",
+		}
+		
+		storage, err := NewStorage(config)
+		assert.Error(t, err)
+		assert.IsType(t, &UnsupportedBackendError{}, err)
+		assert.Nil(t, storage)
+		assert.Contains(t, err.Error(), "unsupported")
+	})
+}
+
+func TestStorageErrors(t *testing.T) {
+	t.Run("UnsupportedBackendError", func(t *testing.T) {
+		err := &UnsupportedBackendError{Backend: "ftp"}
+		assert.Equal(t, "unsupported storage backend: ftp", err.Error())
+	})
+	
+	t.Run("FileNotFoundError", func(t *testing.T) {
+		err := &FileNotFoundError{Path: "missing.txt"}
+		assert.Equal(t, "file not found: missing.txt", err.Error())
+	})
+	
+	t.Run("InvalidPathError", func(t *testing.T) {
+		err := &InvalidPathError{Path: "../../../etc/passwd"}
+		assert.Equal(t, "invalid path: ../../../etc/passwd", err.Error())
+	})
+}