feat: add pushduck as S3-native upload alternative to UploadThing

[abhay-ramesh]

Summary

This PR adds pushduck as a parallel, opt-in upload alternative to the existing UploadThing integration. UploadThing is completely unchanged — both systems coexist and users can choose either.

What changed

New files:

• src/app/api/upload/route.ts — pushduck API route at /api/upload
• src/ui/components/pushduck-upload.tsx — drop-in <ImageUploader /> and <VideoUploader /> React components

Modified files:

• .env.example — adds AWS_S3_* vars section under UPLOADS
• package.json — adds "pushduck": "^0.4.0" dependency

Why pushduck

	UploadThing	pushduck
Storage	UploadThing-managed CDN	Your own S3 / R2 / Spaces / MinIO
Bandwidth cost	Included in plan	Files go browser → S3 directly (presigned URLs) — zero server bandwidth
Vendor lock-in	Yes	None
Route names	imageUploader, videoUploader	Same — imageUploader, videoUploader

Key design decisions

• Presigned URL pattern — the server only signs the upload; bytes travel directly from the browser to S3. No server bandwidth consumed.
• Same route slug names as UploadThing (imageUploader, videoUploader) so swapping is a one-line import change.
• Auth parity — middleware uses auth.api.getSession() identically to core.ts, so session handling is consistent.
• File limits match — 4 MB for images, 64 MB for videos, matching the existing UploadThing config.
• Provider flexibility — the AWS_S3_* vars work with any S3-compatible provider (Cloudflare R2, DigitalOcean Spaces, MinIO) by adjusting the endpoint.

Required env vars (optional — only if using pushduck)

AWS_S3_ACCESS_KEY_ID=
AWS_S3_SECRET_ACCESS_KEY=
AWS_S3_REGION=us-east-1
AWS_S3_BUCKET_NAME=

Test plan

• UploadThing still works at /api/uploadthing (nothing changed)
• <ImageUploader /> from ~/ui/components/pushduck-upload renders and accepts image files
• <VideoUploader /> from ~/ui/components/pushduck-upload renders and accepts video files
• Unauthenticated upload attempt returns 401
• Authenticated upload produces a presigned URL and file lands in the configured S3 bucket
• Progress bar updates during upload, success URL displayed after completion

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