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/model/types/config.go

@@ -1,5 +1,7 @@
 package types
 
+import "github.com/openaccounting/oa-server/core/storage"
+
 type Config struct {
 	WebUrl          string `mapstructure:"weburl"`
 	Address         string `mapstructure:"address"`
@@ -15,8 +17,11 @@ type Config struct {
 	Password        string `mapstructure:"password"`        // Sensitive: use OA_PASSWORD env var
 	// SQLite specific
 	DatabaseFile    string `mapstructure:"databasefile"`
+	// Email configuration
 	MailgunDomain   string `mapstructure:"mailgundomain"`
 	MailgunKey      string `mapstructure:"mailgunkey"`      // Sensitive: use OA_MAILGUN_KEY env var
 	MailgunEmail    string `mapstructure:"mailgunemail"`
 	MailgunSender   string `mapstructure:"mailgunsender"`
+	// Storage configuration
+	Storage         storage.Config `mapstructure:"storage"`
 }