Create a new prediction market (User-facing)

Creates a market in "Pending" status. Admin must activate it before trading.

Request Body

• question: Market question (10-500 chars)
• category: Category (e.g., "crypto", "politics", "sports")
• resolution_time: ISO 8601 datetime when market resolves
• market_type: "binary" (default) or "multi_outcome"
• outcomes: Required for multi_outcome markets

Response

• market_id: On-chain market ID
• slug: URL-friendly identifier
• status: "pending" (requires admin activation)
• tx_signature: Blockchain transaction signature