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/tests/test_main.py

@@ -504,7 +504,7 @@ async def test_concurrent_requests_use_request_scoped_db_connections(tmp_path: P
         transport = ASGITransport(app=app)
         async with AsyncClient(transport=transport, base_url="http://test") as client:
             app.state.setup_complete_cached = True
-            responses = await asyncio.gather(*(client.post("/api/auth/logout") for _ in range(5)))
+            responses = await asyncio.gather(*(client.post("/api/v1/auth/logout") for _ in range(5)))
 
     assert len(connections) == 5
     assert len({id(connection) for connection in connections}) == 5