Confirm TEE wallet delegation AND drain any pending welcome airdrop

Two responsibilities, kept in one round-trip so the frontend can render a single combined success toast:

1. Delegation status. Confirms whether the embedded wallet is currently delegated to our key quorum. The actual delegation is a USER-SIDE action (frontend addSigners()); this endpoint only reads the current state — it does NOT perform the delegation itself.
2. Pending welcome-airdrop drain. Under the deferred-airdrop flow, the welcome airdrop amount is staged at approval time. This endpoint atomically claims that amount, mints the USDC from treasury to the canonical wallet, and clears the staged amount. Idempotent: a second call after a successful drain returns airdrop: null (nothing left to drain) without re-minting. On mint failure, the pending amount is restored so the user can retry on their next call.