feat(api): add /api/stations router with CRUD and assignments

Implements the /api/stations FastAPI router (admin-only CRUD, recipe
assignment endpoints) and the public /by-code/{code}/recipes operator
endpoint. Registers the router in main.py and adds 8 integration tests.

server/main.py

@@ -20,6 +20,7 @@ from routers.settings import router as settings_router
 from routers.reports import router as reports_router
 from routers.statistics import router as statistics_router
 from routers.setup import router as setup_router
+from routers.stations import router as stations_router
 
 
 @asynccontextmanager
@@ -71,6 +72,7 @@ app.include_router(settings_router)
 app.include_router(statistics_router)
 app.include_router(reports_router)
 app.include_router(setup_router)
+app.include_router(stations_router)
 
 
 @app.get("/api/health")