[6.x] Add upload sessions and Uppy transport API

[riasvdv]

Description

Adds a shared upload-session API with ownership checks, expiry cleanup, resumable tus transfers, direct S3 multipart transfers, and asset destination handling. A standalone Uppy client exposes upload, progress, pause/resume, retry, and cancellation through Craft.Uploads.

Browser
  │
  │ create upload session + destination parameters
  ▼
Uploads::start()
  ├─ authorize destination through UploadHandler
  ├─ resolve tempAssetUploadFs
  ├─ choose uploader:
  │    explicit uploader setting, or inferred from temporary filesystem
  ├─ create session with temporary disk, uploader, and destination parameters
  └─ Uploader::start() returns UploadSetup:
       chunkSize + state → saved on session
       transportType + transportOptions → returned to browser
  │
  │ transfer bytes using the selected Uppy transport
  ▼
Temporary upload filesystem
  ├─ Tus: browser → PHP → temporary disk
  └─ S3: browser → temporary S3 disk directly
  │
  │ complete session
  ▼
Uploads::complete()
  ├─ reauthorize destination
  ├─ Uploader::complete() → staged UploadedFile
  ├─ verify file size
  └─ UploadHandler::complete()
       ├─ Assets: copy temporary file → volume filesystem
       └─ User photo: obtain local copy → save photo
  │
  ▼
Uploads calls Uploader::abort() to clean up temporary files

See https://github.com/craftcms/cms/blob/feature/upload-session-transports/docs/uploads.md

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