Step 3: Add admin API endpoints for pricing configuration

- Add PricingConfigResponse and PricingConfigUpdate schemas
- Create PricingService with validation logic
- Add GET and PUT endpoints in routes/pricing.py
- Add MANAGE_PRICING permission to admin role
- Register pricing router in main.py
- Add comprehensive API tests for permissions and validation

backend/schemas/pricing.py

@@ -0,0 +1,27 @@
+from pydantic import BaseModel
+
+
+class PricingConfigResponse(BaseModel):
+    """Response model for pricing configuration."""
+
+    premium_buy: int
+    premium_sell: int
+    small_trade_threshold_eur: int
+    small_trade_extra_premium: int
+    eur_min_buy: int
+    eur_max_buy: int
+    eur_min_sell: int
+    eur_max_sell: int
+
+
+class PricingConfigUpdate(BaseModel):
+    """Request model for updating pricing configuration."""
+
+    premium_buy: int
+    premium_sell: int
+    small_trade_threshold_eur: int
+    small_trade_extra_premium: int
+    eur_min_buy: int
+    eur_max_buy: int
+    eur_min_sell: int
+    eur_max_sell: int