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

prisma/migrations/20260627140000_add_video_upload_multipart_id/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable: track the S3/R2 multipart upload id so chunked uploads can be completed/aborted.
+ALTER TABLE "video_upload_sessions" ADD COLUMN "multipart_upload_id" TEXT;

prisma/schema.prisma

@@ -724,6 +724,7 @@ model VideoUploadSession {
   declaredSizeBytes  BigInt              @map("declared_size_bytes")
   contentType        String              @map("content_type")
   reservationId      String?             @map("reservation_id")
+  multipartUploadId  String?             @map("multipart_upload_id")
   expiresAt          DateTime            @map("expires_at")
   status             UploadSessionStatus @default(INITIATED)
   consumedAt         DateTime?           @map("consumed_at")