Server: no way to route uploads to different buckets per request (keyStrategy doesn't see metadata)

[BSalaeddin]

We're moving an app off a hand-rolled v1 setup that routes each upload to one of three buckets (images, quarantine for unscanned docs, general documents), each with its own credentials and endpoint. The client says which class of upload it is via metadata in the presign request, and the server picks the bucket from that.

With @upupjs/server 3.1.0 this routing can't be expressed:

• storage in UpupServerConfig is a single static object (type/bucket/region/credentials), no function form
• keyStrategy receives { userId, fileName, contentType, size } (dist/config-28bb05f8.d.ts) — no metadata, no request
• hooks.onBeforeUpload does get the Request, but it can only return a boolean, it can't influence where the file goes

So createUpupHandler / createUpupNextHandler only fit single-bucket apps, and we had to keep our own presign route.

Would either of these be acceptable?

1. allow storage to be (ctx) => StorageConfig resolved per request, or
2. pass the presign metadata (and ideally the request) into keyStrategy

Happy to test a canary against a real multi-bucket setup.

[AminDhouib]

Implemented in #353: storage now accepts a per-request resolver (ctx) => StorageConfig | Promise<StorageConfig> (ctx: { req, metadata, userId, fileName, contentType, size } per route), keyStrategy receives metadata + req, and multipart continuations are bound to the resolved destination through the HMAC-signed upload token (a validly-signed token naming storage the resolver won't produce is a 403 with zero storage calls). Provider clients are cached per destination. Your three-bucket case is expressible with either the resolver alone or resolver + metadata. Will close when the PR is promoted to master — a canary test against your real multi-bucket setup would be very welcome.