fix(PM-6095): use forum-style hosted attachment uploads

[jmgasper]

Summary

• keep the browser-to-Support-API authenticated multipart contract
• replace the custom raw S3 Store request with the official Filestack hosted multipart client used by the forum pattern
• avoid all custom storage targeting (bucket/container/path/region/location) and retain canonical CDN response validation
• preserve bounded provider timeouts and safe 502/503 errors

Validation

• pnpm lint
• pnpm build
• pnpm test --runInBand (91 tests)
• Filestack package initialization smoke test

No platform-ui or CSP change is required: the browser never connects to S3 or receives storage credentials.