feat(backend): add deprecation middleware and API versioning support

- Add deprecation middleware for warning headers on sunset endpoints
- Add jails_v2 router for API v2 migration path
- Update CI workflow with new test coverage
- Update API versioning documentation
- Remove completed tasks from Tasks.md

backend/app/routers/jails_v2.py

@@ -0,0 +1,40 @@
+"""Jails router — v2 (pre-production).
+
+This router contains the next-generation version of the jails API,
+intended for the v2 breaking-change release. It is registered in
+``app/main.py`` but is NOT active for production traffic until the
+v2 launch milestone is completed.
+
+To activate v2 for a specific endpoint, move the handler from the v1
+router to this router and remove the ``include_in_schema=False`` flag.
+
+Until then, all endpoints here return ``404 Not Found`` so they do not
+interfere with live v1 traffic.
+"""
+
+from __future__ import annotations
+
+from fastapi import APIRouter, status
+from fastapi.responses import JSONResponse
+
+#: Set to ``True`` once v2 is declared production-ready.
+_V2_ENABLED: bool = False
+
+router: APIRouter = APIRouter(prefix="/api/v2/jails", tags=["Jails (v2)"])
+
+
+@router.get(
+    "",
+    include_in_schema=False,
+)
+async def jails_v2_placeholder() -> JSONResponse:
+    """Placeholder handler — v2 not yet enabled.
+
+    Returns 404 so this router never serves real traffic until v2 is
+    explicitly activated. When v2 goes live, remove this handler and
+    expose the real endpoints.
+    """
+    return JSONResponse(
+        status_code=status.HTTP_404_NOT_FOUND,
+        content={"detail": "v2 endpoint not yet available"},
+    )