login: honour next on both exits, same-origin only

The console's 401 redirect carries next=/leaders/ and /login dropped it, so a
leader arriving cold landed on /account. next now rides the form as a hidden
field and is applied after a successful POST and when an already-signed-in
person hits /login. identity.safe_next() admits only a relative path with a
single leading slash - no scheme, no //, no backslash, no control characters -
so the login page cannot become an open redirect. Ten checks added to
tests/smoke_identity.py; the suite is 59 + 24 + 53, all green.

app/auth.py

@@ -82,10 +82,11 @@ def _shell(heading, intro, inner, error=None):
 # Login
 # ---------------------------------------------------------------------------
 
-def _login_form(email="", error=None):
+def _login_form(email="", error=None, next_url=""):
     return _shell(
         "Sign in", "For Pack 73 and Troop 73 leaders and families.",
         f"""<form method="post" action="/login">
+<input type="hidden" name="next" value="{_esc(next_url)}">
 <label for="email">Email</label>
 <input id="email" name="email" type="email" autocomplete="username" required value="{_esc(email)}">
 <label for="password">Password</label>
@@ -98,21 +99,26 @@ def _login_form(email="", error=None):
 
 @router.get("/login", response_class=HTMLResponse)
 def login_form(request: Request):
+    # `next` arrives from the leader console's 401 redirect (next=/leaders/).
+    # identity.safe_next() keeps it same-origin; anything odd lands on /account.
+    nxt = identity.safe_next(request.query_params.get("next"))
     if current_person(request):
-        return RedirectResponse(url="/account", status_code=303)
-    return HTMLResponse(_render("Sign in", _login_form()))
+        return RedirectResponse(url=nxt, status_code=303)
+    return HTMLResponse(_render("Sign in", _login_form(next_url=nxt)))
 
 
 @router.post("/login")
-def login(request: Request, email: str = Form(""), password: str = Form("")):
+def login(request: Request, email: str = Form(""), password: str = Form(""),
+          next: str = Form("")):
+    nxt = identity.safe_next(next)
     try:
         person, token = identity.authenticate(
             email, password, ip=_client_ip(request),
             user_agent=request.headers.get("user-agent"))
     except identity.IdentityError as e:
-        return HTMLResponse(_render("Sign in", _login_form(email, e.detail)),
+        return HTMLResponse(_render("Sign in", _login_form(email, e.detail, nxt)),
                             status_code=e.status)
-    resp = RedirectResponse(url="/account", status_code=303)
+    resp = RedirectResponse(url=nxt, status_code=303)
     resp.set_cookie(COOKIE, token, max_age=identity.SESSION_ABSOLUTE_DAYS * 86400,
                     httponly=True, secure=COOKIE_SECURE, samesite="lax", path="/")
     return resp