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.go

@@ -0,0 +1,106 @@
+package storage
+
+import (
+	"io"
+	"time"
+)
+
+// Storage defines the interface for file storage backends
+type Storage interface {
+	// Store saves a file and returns the storage path/key
+	Store(filename string, content io.Reader, contentType string) (string, error)
+	
+	// Retrieve gets a file by its storage path/key
+	Retrieve(path string) (io.ReadCloser, error)
+	
+	// Delete removes a file by its storage path/key
+	Delete(path string) error
+	
+	// GetURL returns a URL for accessing the file (may be signed/temporary)
+	GetURL(path string, expiry time.Duration) (string, error)
+	
+	// Exists checks if a file exists at the given path
+	Exists(path string) (bool, error)
+	
+	// GetMetadata returns file metadata (size, last modified, etc.)
+	GetMetadata(path string) (*FileMetadata, error)
+}
+
+// FileMetadata contains information about a stored file
+type FileMetadata struct {
+	Size         int64
+	LastModified time.Time
+	ContentType  string
+	ETag         string
+}
+
+// Config holds configuration for storage backends
+type Config struct {
+	// Storage backend type: "local", "s3"
+	Backend string `mapstructure:"backend"`
+	
+	// Local filesystem configuration
+	Local LocalConfig `mapstructure:"local"`
+	
+	// S3-compatible storage configuration (S3, B2, R2, etc.)
+	S3 S3Config `mapstructure:"s3"`
+}
+
+// LocalConfig configures local filesystem storage
+type LocalConfig struct {
+	// Root directory for file storage
+	RootDir string `mapstructure:"root_dir"`
+	
+	// Base URL for serving files (optional)
+	BaseURL string `mapstructure:"base_url"`
+}
+
+// S3Config configures S3-compatible storage (AWS S3, Backblaze B2, Cloudflare R2, etc.)
+type S3Config struct {
+	// AWS Region (use "auto" for Cloudflare R2)
+	Region string `mapstructure:"region"`
+	
+	// S3 Bucket name
+	Bucket string `mapstructure:"bucket"`
+	
+	// Optional prefix for all objects
+	Prefix string `mapstructure:"prefix"`
+	
+	// Access Key ID
+	AccessKeyID string `mapstructure:"access_key_id"`
+	
+	// Secret Access Key
+	SecretAccessKey string `mapstructure:"secret_access_key"`
+	
+	// Custom endpoint URL for S3-compatible services:
+	// - Backblaze B2: https://s3.us-west-004.backblazeb2.com
+	// - Cloudflare R2: https://<account-id>.r2.cloudflarestorage.com
+	// - MinIO: http://localhost:9000
+	// Leave empty for AWS S3
+	Endpoint string `mapstructure:"endpoint"`
+	
+	// Use path-style addressing (required for some S3-compatible services)
+	PathStyle bool `mapstructure:"path_style"`
+}
+
+
+// NewStorage creates a new storage backend based on configuration
+func NewStorage(config Config) (Storage, error) {
+	switch config.Backend {
+	case "local", "":
+		return NewLocalStorage(config.Local)
+	case "s3":
+		return NewS3Storage(config.S3)
+	default:
+		return nil, &UnsupportedBackendError{Backend: config.Backend}
+	}
+}
+
+// UnsupportedBackendError is returned when an unknown storage backend is requested
+type UnsupportedBackendError struct {
+	Backend string
+}
+
+func (e *UnsupportedBackendError) Error() string {
+	return "unsupported storage backend: " + e.Backend
+}