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/api/routes.go

@@ -36,6 +36,12 @@ func GetRouter(auth *AuthMiddleware, prefix string) (rest.App, error) {
 		rest.Get(prefix+"/orgs/:orgId/transactions/:transactionId/attachments/:attachmentId", auth.RequireAuth(GetAttachment)),
 		rest.Get(prefix+"/orgs/:orgId/transactions/:transactionId/attachments/:attachmentId/download", auth.RequireAuth(DownloadAttachment)),
 		rest.Delete(prefix+"/orgs/:orgId/transactions/:transactionId/attachments/:attachmentId", auth.RequireAuth(DeleteAttachment)),
+		
+		// New storage-based attachment endpoints
+		rest.Post(prefix+"/attachments", auth.RequireAuth(PostAttachmentWithStorage)),
+		rest.Get(prefix+"/attachments/:id", auth.RequireAuth(GetAttachmentWithStorage)), 
+		rest.Get(prefix+"/attachments/:id/url", auth.RequireAuth(GetAttachmentDownloadURL)),
+		rest.Delete(prefix+"/attachments/:id", auth.RequireAuth(DeleteAttachmentWithStorage)),
 		rest.Get(prefix+"/orgs/:orgId/prices", auth.RequireAuth(GetPrices)),
 		rest.Post(prefix+"/orgs/:orgId/prices", auth.RequireAuth(PostPrice)),
 		rest.Delete(prefix+"/orgs/:orgId/prices/:priceId", auth.RequireAuth(DeletePrice)),