Replaces dead db_get mock (never consumed by the render step) with a
proper aiohttp.ClientSession mock. The render call uses aiohttp directly,
so the old mock hit the non-fatal except branch instead of the 200-OK path.
Adds explicit assertion for '✅ Card rendered + S3 upload triggered'.
Closes#169
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>