chore(api): drop unused KMS dependency and config

[MehrshadFb]

Summary

Removes AWS KMS remnants that were provisioned for photo encryption but never used. Photos are encrypted at rest by S3's default SSE-S3 (AES-256); after review we decided customer-managed KMS keys are not needed at this stage — they add per-request cost and an operational failure mode without a compliance driver. If that changes later, SSE-KMS is a bucket-level setting requiring no API code.

What's included

• Uninstall @aws-sdk/client-kms (never imported anywhere)
• Drop the unused kmsKeyId entry from aws.config.ts
• Remove AWS_KMS_KEY_ID from .env.example

Not in this PR (intentional)

• ENCRYPTION_MASTER_KEY / encryption.config.ts and MAX_FILE_SIZE are also dead config from the same era (values never read) — candidates for a small follow-up

Test plan

• No KMS references remain in api/src, api/test, env files, or package.json
• npm run build, npm test (265), npm run test:e2e (101), eslint, format:check all green