Add Static Slots To Billing Plan

Make static slots a feature on the billing plan. Default: 0 When 0 user should not see any controls for adding static worktrees. Remove words "static" / "dynamic" from worktrees UI. Draw a little icon for static (like a pin) and nothing for dynamic.

Static slots plan feature (default 0 hides all add controls, server gate), pin icon replaces Static/Dynamic. Full gate green; #4708 merged, migration-order fixes #4710/#4711 merged; Jaah Deploy green. Deferred: node-group worktrees_per_node not gated.