feat: add S3-compatible storage provider

[onyedikachi-david]

What kind of change does this PR introduce?

Feature.

Adds AWS S3 and S3-compatible storage provider support, including MinIO-compatible endpoint configuration.

Why was this change needed?

Fixes #1124.

Postiz already supports local and Cloudflare R2 storage, but self-hosted deployments may use AWS S3 or S3-compatible providers such as MinIO, DigitalOcean Spaces, or Backblaze B2. This change adds an s3 storage provider so those deployments can upload, save, preview, and serve media through S3-compatible object storage.

This PR adds:

• S3 storage provider implementation for simple/server-side uploads.
• S3 multipart upload support using Uppy’s existing S3-compatible multipart flow.
• S3_ENDPOINT support with path-style requests for MinIO/S3-compatible providers.
• Frontend storage-provider wiring so STORAGE_PROVIDER=s3 uses S3 upload and preview URLs.
• Example S3 environment variables in .env.example.

Other information:

Validated locally with AWS S3 media uploads and preview rendering.

Checklist:

• I have read the CONTRIBUTING guide.
• I have signed the Contributor License Agreement (CLA) (ICLA for individuals, CCLA for entities).
• I confirm I have not used AI to submit this PR or generate code for it.
• I checked that there were no similar issues or PRs already open for this.
• This PR fixes just ONE issue

[postiz-agent]

✅ Snyk checks have passed. No issues have been found so far.

Status	Scan Engine	Critical	High	Medium	Low	Total (0)
✅	Open Source Security	0	0	0	0	0 issues
✅	Licenses	0	0	0	0	0 issues
✅	Code Security	0	0	0	0	0 issues

💻 Catch issues earlier using the plugins for VS Code, JetBrains IDEs, Visual Studio, and Eclipse.

[onyedikachi-david]

this PR is ready for review but I can't remove from draft @egelhaus