#15423 Grant meter credits from the runtime SDK

Stacked on #15420. Uses the credits endpoint from #15405 and first requires https://github.com/polarsource/polar/pull/15440 to be merged.

Adds grant to the runtime SDK actor, so a merchant can give a customer credits from their server without a purchase:

await polar.actor({ externalCustomerId: userId }).grant('tool_call', 50, {
  idempotencyKey: `survey:${userId}`,
})

• The meter name is type-checked against the config. rollover defaults to true.
• idempotencyKey maps to the endpoint's external_id, so retrying doesn't grant twice.
• The balance updates asynchronously, like it does for tracked events so the next balance() reports pristine: false until the credit is applied.
• Credits always go to the customer, even on an actor with a member.

The demo app shows: a free customer who runs out of tool calls can answer "What are you building?" to get 10 tool calls instead of upgrading. The code panel has a new Grant step.

Screenshots