feat: add FETCH_PRICE permission for manual price fetch endpoint

The POST /api/audit/price-history/fetch endpoint now requires
FETCH_PRICE permission instead of VIEW_AUDIT, which is more
semantically correct since it's a write operation.

backend/routes/audit.py

@@ -190,7 +190,7 @@ async def get_price_history(
 @router.post("/price-history/fetch", response_model=PriceHistoryResponse)
 async def fetch_price_now(
     db: AsyncSession = Depends(get_db),
-    _current_user: User = Depends(require_permission(Permission.VIEW_AUDIT)),
+    _current_user: User = Depends(require_permission(Permission.FETCH_PRICE)),
 ) -> PriceHistoryResponse:
     """Manually trigger a price fetch from Bitfinex."""
     price, timestamp = await fetch_btc_eur_price()