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_routers/test_dependency_injection.py

@@ -146,7 +146,7 @@ async def test_auth_login_uses_injected_auth_service(tmp_path: Path) -> None:
         base_url="http://test",
     ) as client:
         response = await client.post(
-            "/api/auth/login",
+            "/api/v1/auth/login",
             json={"password": "ignored"},
         )
 
@@ -185,7 +185,7 @@ async def test_jail_list_uses_injected_jail_service_and_auth(tmp_path: Path) ->
         base_url="http://test",
     ) as client:
         response = await client.get(
-            "/api/jails",
+            "/api/v1/jails",
             headers={"Cookie": f"{SESSION_COOKIE_NAME}=fake-token"},
         )