fix(ST-55): explain film creation limits

[afaryy]

Summary

• show clear, senior-friendly messages for device, network, shared, concurrency, and original-song limits
• keep billing details and the phrase “cost control” out of the user-facing experience
• return a short Retry-After for busy/concurrency gates and a next-day retry interval for daily gates

Validation

• API: 206 tests passed
• Web: 53 tests passed
• Next.js production build passed
• git diff --check passed
• independent code review found no critical, important, or minor issues

Scope

• Implements ST-55
• Does not include the separate ST-56 request-field visual change
• Keeps the conservative production defaults unchanged: 10 films per IP/day, 5 films per visitor/day, and 3 original songs per visitor/day