fix(files): validate upload metadata map

[aiirvizionz]

Summary

• reject invalid encryptedMetadata before profile and conversation lookups
• prevent strings or arrays from being treated as recipient metadata maps
• add focused regression coverage for invalid encrypted metadata

Test

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

uGig billable bugfix candidate. Not counted as revenue until accepted, invoiced, and paid.