After each successful image upload to storage, a sidecar `.caption`
file is uploaded at the same path with `.caption` extension containing
the exact prompt used to generate the image.
Coverage:
- generation/handlers.go: ImageHandler → media/{userID}/images/{jobID}_{i}.caption
- album/handler.go: AnchorHandler → albums/{userID}/{albumID}/anchor.caption
- album/handler.go: ShotHandler → albums/{userID}/{albumID}/shots/{shotIndex}.caption
- personagen/service.go: generatePosition → personas/{specID}/images/{pos:02d}.caption
Caption failures are logged at warn level and never abort the job.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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