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_utils/test_global_rate_limiter.py

@@ -17,7 +17,7 @@ _SETUP_PAYLOAD = {
 
 async def _do_setup(client: AsyncClient) -> None:
     """Run the setup wizard so auth endpoints are reachable."""
-    resp = await client.post("/api/setup", json=_SETUP_PAYLOAD)
+    resp = await client.post("/api/v1/setup", json=_SETUP_PAYLOAD)
     assert resp.status_code == 201
 
 
@@ -146,11 +146,11 @@ class TestRateLimitMiddleware:
         try:
             # First 3 requests should succeed
             for i in range(3):
-                response = await client.get("/api/health")
+                response = await client.get("/api/v1/health")
                 assert response.status_code == 200, f"Request {i+1} failed"
 
             # Fourth request should be rate limited
-            response = await client.get("/api/health")
+            response = await client.get("/api/v1/health")
             assert response.status_code == 429
             assert response.json()["code"] == "rate_limit_exceeded"
             assert "Retry-After" in response.headers
@@ -169,11 +169,11 @@ class TestRateLimitMiddleware:
 
         try:
             # First request succeeds
-            response = await client.get("/api/health")
+            response = await client.get("/api/v1/health")
             assert response.status_code == 200
 
             # Second request is rate limited
-            response = await client.get("/api/health")
+            response = await client.get("/api/v1/health")
             assert response.status_code == 429
             assert "Retry-After" in response.headers
             retry_after = int(response.headers["Retry-After"])