Add blog post on commercial use of the Metron API

[bpepple]

Summary

Adds a blog post (blog/2026-10-04-commercial-api-use/) that answers the questions app developers, comic shops, and other businesses have been e-mailing about using the Metron API in their products:

• Commercial use: allowed under CC BY-SA 4.0. Metron being run by volunteers doesn't restrict what the data can be used for.
• Accounts and rate limits: apps used by many people need one Metron account per user. A business using the data for itself, such as updating its online store, uses one account. The post covers the standard limits, caching, API tokens, the rate limit headers, spreading out scheduled jobs, and supporter tiers for more capacity.
• Attribution and ShareAlike: gives example attribution text. Explains that ShareAlike applies when adapted data is published or distributed, not when it's only stored and used internally.
• Cover images: they aren't covered by the CC license. Metron hosts them as fair use, and other sites need to judge their own use or get permission from publishers.
• Supporting the project: Open Collective donations, including the supporter-rate-limit and no-Incognito notes.

The post doesn't name the businesses that wrote in.