feat: S3-compatible object storage backend

BACKEND_STORAGE_DRIVER=s3 swaps LocalStore for a stdlib-only SigV4
implementation — PutObject/GetObject/DeleteObject/CopyObject/ListObjectsV2
against any S3-compatible API (AWS, MinIO, Garage, R2). Storage becomes a
Store interface; storage_path records s3://bucket/key. Content-Length is
resolved by seeking or temp-file spooling — chunked uploads are rejected
by real S3 servers. Bucket auto-created on startup.

Verified live against zenko/cloudserver: bucket, put, get, copy, delete.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/internal/storage/store.go

@@ -0,0 +1,16 @@
+package storage
+
+import (
+	"context"
+	"io"
+)
+
+// Store is the object-storage contract the platform service depends on.
+// LocalStore keeps objects on disk; S3Store targets S3-compatible APIs.
+type Store interface {
+	Put(ctx context.Context, bucketID, objectKey string, reader io.Reader) (PutResult, error)
+	Open(bucketID, objectKey string) (io.ReadCloser, string, error)
+	Delete(bucketID, objectKey string) error
+	MoveBetweenBuckets(sourceBucketID, destinationBucketID, fromKey, toKey string) (string, error)
+	DeleteBucket(bucketID string) error
+}