GET /api/v1/predictions/me/positions/

Return all of the authenticated user's prediction market positions, enriched with current LSMR value and P&L.

Auth: Uses DRF's DEFAULT_AUTHENTICATION_CLASSES (JWTCookieAuthentication + JWTAuthentication) which supports both cookie and Bearer token auth without CSRF enforcement — appropriate for a read-only endpoint.

Query params: market_status — filter positions by their market's status (default: 'open'; pass 'all' for every market)

Response: { "results": [ <PositionSerializer>, ... ], "count": <int> }