docs(middles): rewrite OpenAPI spec — fix endpoints, params, and schemas

- Fix endpoint path /middles → /opportunities/middles
- Add /opportunities/middles/summary and /opportunities/middles/{id} endpoints
- Fix param names (sports→sport, min_gap→min_size, market_type→market)
- Add 8 missing params: league, sportsbook, max_odds_age, live, sort, offset, state
- Rewrite MiddleOpportunity schema with all ~30 fields in snake_case
- Add MiddleSide, MiddleSideOdds, GameState component schemas
- Fix tier requirement: Sharp → Pro

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Mlaz-code