feat(ST-38): generate safe original memory songs

[afaryy]

Summary

• Add a consent-scoped original-memory-song path using Lyria 3 with Vertex AI service-account authentication in Cloud Run.
• Reject requests for artist, song, or voice imitation; offer instrumental and no-music fallbacks.
• Mix temporary generated audio into fixed 60-second photo/video films, looping as needed and fading out safely.
• Expose simple soundtrack choices in the web flow and document the hosted-verification boundary.

Verification

• API suite: 59 passed. One existing Starlette/httpx deprecation warning remains.
• Web suite: 16 passed.
• Production web build: passed.

Hosted verification still required

Lyria 3 is a preview service. This PR is locally implemented and tested, but it is not production or competition evidence until a non-sensitive hosted smoke test produces a playable generated-audio render.