feat: chunked (S3 multipart) uploads for R2/S3 video backend

Self-hosted instances on the R2/S3 backend could only upload a video as a
single PUT, which fails behind a Cloudflare proxy/tunnel (100MB request-body
cap) and is capped at 5GiB with no resilience. Bunny already avoids this via
tus; this brings the R2/S3 path to parity.

Files larger than a threshold (default 90MiB) are now split into parts
(default 32MiB, min 5MiB) and uploaded directly browser->R2 via presigned
UploadPart URLs, then reassembled server-side with CompleteMultipartUpload.
Each request stays under the 100MB cap, lifts the size ceiling well past
5GiB, and adds per-chunk retry. Files at/under the threshold keep the
existing single-PUT path unchanged. Bunny path is untouched.

Thresholds are env-overridable via OPENFRAME_R2_MULTIPART_THRESHOLD_BYTES
and OPENFRAME_R2_MULTIPART_PART_SIZE_BYTES.

Verified end-to-end against real Cloudflare R2 and a local MinIO behind an
nginx 90MB cap (single 141MB PUT 413s on master; 32MB parts pass here).

Closes #22

lib/r2-upload-session.ts

@@ -11,6 +11,7 @@ export type CreateR2UploadSessionInput = {
   reservationId: string | null;
   uploadJti: string;
   expiresAt: Date;
+  multipartUploadId?: string | null;
 };
 
 export async function createR2UploadSession(input: CreateR2UploadSessionInput) {
@@ -26,6 +27,7 @@ export async function createR2UploadSession(input: CreateR2UploadSessionInput) {
       reservationId: input.reservationId,
       uploadJti: input.uploadJti,
       expiresAt: input.expiresAt,
+      multipartUploadId: input.multipartUploadId ?? null,
     },
   });
 }