Provision CLOB credentials for an imported Polymarket wallet

The equivalent of POST /exchanges/polymarket/enable-trading for a wallet imported from polymarket.com. Derives and stores CLOB credentials against the imported EOA. No approval legs run — an imported wallet already carries its on-chain approvals from its prior polymarket.com activity. Neither wallet_address nor turnkey_org_id is trusted input: both are checked against the authenticated caller before anything is minted, so a stolen session cannot provision credentials against a wallet or sub-org it has no relationship to. Rate limited at 5 requests/minute per user (CLOB_PROVISIONING_RATE_LIMIT_PER_MIN, bucket clob-provisioning), fail-closed — each allowed call makes a real CLOB round-trip and writes a credential row. The 429 carries error_details.code = VALIDATION_INVALID_ORDER with the message Too many import attempts — please wait a moment and try again. Auth & scope. Requires trade:execute for polymarket and the mutation gate.