fix(files): reject empty upload metadata

[aiirvizionz]

Summary

• reject empty encryptedMetadata maps during upload completion
• prevent file records from being saved without any recipient metadata copy
• add regression coverage for empty encrypted metadata payloads

Test

• node node_modules\vitest\vitest.mjs run --config %TEMP%\qryptchat-vitest-minimal.config.mjs "src/app/api/files/upload-complete/route.test.js"
• git diff --check

uGig billing

• Expected invoice: 0.50 USD if accepted/merged
• Counted as revenue now: no, pending review/payment