Plans are a setting: free, small, medium and large, each with a size, a monthly price, a monthly crew credit and a forge count #11135

Why. Tiers and ceilings bound free forges as well as paid ones. Limits are settings on a page, never constants.

What. A plans setting an admin edits as a form (and by API/MCP): for each plan a key, a name, one line of words, the forge size (src/spawn/sizes.ts), price in USD per month (0 = free), crew credit in USD per month (the child's credit cap), and how many forges one person may hold on it.

Defaults (the owner changes them on the page):

• A spawn takes a plan; the forge's size and credit cap come from it. The free plan keeps today's visitor and per-person limits (src/spawn/rate.ts, forges.per_person, forges.storage_mb); a paid plan is not held to the free count.
• The credit cap resets each billing period (each calendar month under none).
• GET /api/v1/plans is public and answers the plans without secrets.

Acceptance. Changing a price or a credit on the settings page changes what /api/v1/plans answers and what the next spawn gets, with no restart. Tests cover the defaults, an edit, and a spawn on each plan.