fix: eliminate duplicate uploads and wasted credits in ref image gen

[crisng95]

Summary

• Direct UUID extraction: After batchGenerateImages, extract UUID from response before uploading. If found, use it directly — no duplicate image in Google Flow
• Skip regen on retry: If reference_image_url exists but media_id is missing, only retry the upload (saves credits)
• SSL fix: macOS Python lacks root certs for storage.googleapis.com, causing _upload_character_image to always fail with SSLCertVerificationError

Problem

Previously each entity generated 2 images in Google Flow:

1. Generated image (from batchGenerateImages) — named from prompt
2. Re-uploaded copy (from uploadImage) — named "Viktor.jpeg"

On upload failure, the worker retried the entire generation (up to 5x), burning credits each time.

Flow after fix

Generate → UUID in response? → YES → use it (1 image, no duplicate)
                              → NO  → upload copy (2 images, fallback only)
Retry    → image exists?     → YES → upload only (no regen, no credit)

Test plan

• Generate ref image — verify only 1 image appears in Google Flow (not 2)
• Verify media_id is valid UUID and works as imageInputs in scene image gen
• Simulate upload failure — verify retry doesn't regenerate image

Supersedes #1

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