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/conftest.py

@@ -16,6 +16,7 @@ from httpx import ASGITransport, AsyncClient
 from app.config import Settings
 from app.db import init_db
 from app.main import create_app
+from app.models.server import ServerStatus
 
 
 @pytest.fixture
@@ -63,6 +64,10 @@ async def client(test_settings: Settings) -> AsyncClient:  # type: ignore[misc]
     """
     app = create_app(settings=test_settings)
 
+    # Ensure fail2ban is reported as online for tests (mock socket is not
+    # actually connected so we need to set the cached status manually).
+    app.state.server_status = ServerStatus(online=True)
+
     # Bootstrap the database schema before making requests.  ASGITransport
     # does not run the application lifespan, so we create the test SQLite file
     # directly rather than relying on startup logic.