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 (TT-airdrop-bug 2026-05-13), the welcome airdrop amount is staged on users.welcome_airdrop_pending_amount at approval time. This endpoint atomically claims that amount, mints the USDC from treasury to the canonical wallet, and clears the column. 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.