Open a pot with frozen verification cases

Without paying, for the key that created the request only. A waiting pot that no key asked for is opened by the first person who adds money with its contract (POST /v1/bounties/{id}/pledges with contract); this route answers 403 with pledge_url there. Send at most 64KB of JSON with public and hidden cases and a scoring rubric. At least 3 public and 5 hidden cases are required. File tables use key_column and expected field objects; whole-text cases use null input and normalized or contains_all comparison. Endpoint cases must satisfy input_schema and output_schema. The same contract succeeds on retry; changing a frozen contract returns 409. Hidden inputs are sent to an endpoint when it is checked.