Fix hosted offer availability in structured data

[gregagi]

Summary

• Declare https://schema.org/OnlineOnly availability for both hosted offers (7-day trial and Pro) in homepage SoftwareApplication and pricing Product JSON-LD.
• Preserve prices, billing durations and schema types. Do not invent ratings or reviews: Google explicitly allows Product snippets with offers alone and warns about the optional missing review fields.

Verification

• Confirmed live homepage and pricing markup omit availability; checked Google's supported Offer enumeration and Product requirements.
• git diff --check passed. Full existing backend/frontend/CLI checks run in PR CI (Docker is unavailable in this runtime).
• Verify both public pages after deployment.
• No changelog entry per repo AGENTS.md exclusion for SEO work.

Reference: https://developers.google.com/search/docs/appearance/structured-data/product-snippet