Phase 2.1-2.3: Add exchange endpoints

Add exchange trading endpoints:
- POST /api/exchange: Create exchange trade
  - Validates slot, price staleness, EUR amount limits
  - Calculates sats from EUR and agreed price
  - Direction-specific premium (buy=+5%, sell=-5%)

- GET /api/trades: List user's exchanges
- POST /api/trades/{id}/cancel: Cancel user's exchange

Add schemas:
- ExchangeRequest, ExchangeResponse
- ExchangeUserContact, AdminExchangeResponse (for Phase 2.4)
- PaginatedExchanges, PaginatedAdminExchanges

backend/main.py

@@ -41,7 +41,6 @@ app.add_middleware(
 # Include routers - modules with single router
 app.include_router(auth_routes.router)
 app.include_router(audit_routes.router)
-app.include_router(exchange_routes.router)
 app.include_router(profile_routes.router)
 app.include_router(availability_routes.router)
 app.include_router(meta_routes.router)
@@ -51,3 +50,5 @@ for r in invites_routes.routers:
     app.include_router(r)
 for r in booking_routes.routers:
     app.include_router(r)
+for r in exchange_routes.routers:
+    app.include_router(r)