feat: save prompt caption alongside every generated image

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>

internal/adapter/templates/templates/skeleton/pkg/album/handler.go.tmpl

@@ -83,6 +83,10 @@ func AnchorHandler(mg *mediagen.Manager, store storage.Store, pub realtime.Event
 				logger.Warn("failed to persist anchor to storage, using inline URL", "error", uploadErr, "job_id", job.ID)
 			} else {
 				anchorURL = url
+				captionPath := fmt.Sprintf("albums/%s/%s/anchor.caption", userID, albumID)
+				if _, captionErr := store.Upload(ctx, captionPath, []byte(subjectDesc), "text/plain"); captionErr != nil {
+					logger.Warn("failed to persist anchor caption", "error", captionErr, "job_id", job.ID)
+				}
 			}
 		}
 
@@ -213,6 +217,10 @@ func ShotHandler(mg *mediagen.Manager, store storage.Store, pub realtime.EventPu
 					"error", uploadErr, "job_id", job.ID)
 			} else {
 				imageURL = url
+				captionPath := fmt.Sprintf("albums/%s/%s/shots/%d.caption", userID, albumID, shotIndex)
+				if _, captionErr := store.Upload(ctx, captionPath, []byte(prompt), "text/plain"); captionErr != nil {
+					logger.Warn("failed to persist shot caption", "error", captionErr, "job_id", job.ID)
+				}
 			}
 		}
 

internal/adapter/templates/templates/skeleton/pkg/generation/handlers.go.tmpl

@@ -100,6 +100,10 @@ func ImageHandler(mg *mediagen.Manager, store storage.Store, pub realtime.EventP
 				if uploadErr != nil {
 					logger.Warn("failed to persist image to storage", "error", uploadErr, "job_id", job.ID)
 				} else {
+					captionPath := fmt.Sprintf("media/%s/images/%s_%d.caption", userID, job.ID, i)
+					if _, captionErr := store.Upload(ctx, captionPath, []byte(prompt), "text/plain"); captionErr != nil {
+						logger.Warn("failed to persist image caption", "error", captionErr, "job_id", job.ID)
+					}
 					images[i] = GeneratedImage{Data: url, IsURL: true, Seed: resp.Seed}
 					continue
 				}

internal/adapter/templates/templates/skeleton/pkg/personagen/service.go.tmpl

@@ -228,6 +228,13 @@ func (s *Service) generatePosition(ctx context.Context, spec *persona.PersonaSpe
 		return fmt.Errorf("storing position %d: %w", pos, err)
 	}
 
+	if imgSpec.Prompt != "" {
+		captionPath := fmt.Sprintf("personas/%s/images/%02d.caption", spec.ID, pos)
+		if _, captionErr := s.store.Upload(ctx, captionPath, []byte(imgSpec.Prompt), "text/plain"); captionErr != nil {
+			s.logger.Warn("failed to persist image caption", "error", captionErr, "position", pos)
+		}
+	}
+
 	imgSpec.URL = url
 	imgSpec.Status = persona.ImageStatusComplete
 	return nil