feat(media): S3/MinIO storage adapter (StoragePort)

Drop-in S3-compatible backend for object storage (media + email attachments) —
the swap the StoragePort seam was designed for. MinIO/self-hosted/R2/Supabase all
work via endpoint + path-style.

- S3Storage implements StoragePort (put/get/remove); sha256 computed locally on put
  (matches LocalDiskStorage); a missing object reads back as null, not an error.
- Env-driven binding in MediaModule: IIOS_S3_BUCKET + keys set → S3Storage, else
  LocalDiskStorage. forcePathStyle defaults true (MinIO); endpoint omitted → AWS.
- S3 client is injectable so tests run with no live server.

6 unit tests (config parse, put size/sha, get round-trip, NoSuchKey→null, remove).
Full suite 300/300, boundary + build clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/iios-service/package.json

@@ -12,6 +12,7 @@
     "prisma:studio": "prisma studio"
   },
   "dependencies": {
+    "@aws-sdk/client-s3": "^3.1090.0",
     "@insignia/iios-adapter-sdk": "workspace:*",
     "@insignia/iios-contracts": "workspace:*",
     "@nestjs/common": "^11.1.27",