fix(files): validate upload metadata values

[aiirvizionz]

Summary

• reject non-string or blank encryptedMetadata values before upload-url database work
• prevent malformed recipient metadata maps from reaching profile/conversation lookups
• add regression coverage for object and whitespace-only metadata values

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 billing

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