feat: implement API versioning /api/v1/

- All backend routers moved to /api/v1/ prefix
- Frontend BASE_URL updated to /api/v1
- Setup redirect middleware updated to redirect to /api/v1/setup
- Health router path fixed: prefix=/api/v1/health, @router.get('')
- conftest.py: set server_status=online for test fixture
- Created Docs/API_VERSIONING.md with deprecation policy
- Updated Docs/Backend-Development.md with versioning section
- Updated Instructions.md curl examples

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/routers/config.py

@@ -4,7 +4,7 @@ from fastapi import APIRouter
 
 from app.routers import action_config, config_misc, filter_config, jail_config
 
-router: APIRouter = APIRouter(prefix="/api/config", tags=["Config"])
+router: APIRouter = APIRouter(prefix="/api/v1/config", tags=["Config"])
 
 router.include_router(jail_config.router)
 router.include_router(filter_config.router)