fix: enforce Literal validation on sort parameter in GET /api/v3/players (#66)

Closes #66

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

app/routers_v3/players.py

@@ -4,7 +4,7 @@ Thin HTTP layer using PlayerService for business logic.
 """
 
 from fastapi import APIRouter, Query, Response, Depends
-from typing import Optional, List
+from typing import Literal, Optional, List
 
 from ..dependencies import oauth2_scheme, cache_result, handle_db_errors
 from ..services.base import BaseService
@@ -23,7 +23,7 @@ async def get_players(
     pos: list = Query(default=None),
     strat_code: list = Query(default=None),
     is_injured: Optional[bool] = None,
-    sort: Optional[str] = None,
+    sort: Optional[Literal["cost-asc", "cost-desc", "name-asc", "name-desc"]] = None,
     limit: Optional[int] = Query(
         default=None, ge=1, description="Maximum number of results to return"
     ),