The switch knows who still uses legacy mail apps (LP-15, server)

The impact panel's data. Every successful sign-in over IMAP, POP3,
ManageSieve or SMTP AUTH records, per account and per protocol, one
timestamp -- nothing else: no address, no IP, no client. It is written at
most once an hour per account and protocol, so a mail app polling every
minute costs a read per sign-in and a write an hour. A record that can't be
written is logged and the sign-in goes ahead.

Both switches serve it as a read-only property, recentLegacyUse, as
wouldClose serves the confirmation: a list of {accountId, name, protocol,
lastUsedAt} for sign-ins in the last 30 days, most recent first.
inbuxa:ProtocolPolicy lists every account; inbuxa:TenantProtocolPolicy
lists only its tenant's own (MT-1). Accounts since deleted are left out. It
is computed only when the property is asked for.

The recording sits where the tenant check already runs once the account is
known, which becomes admit_legacy_session: refuse if the account's tenant
has legacy protocols off, otherwise record. A refused sign-in is never
recorded.

The spec leaves the interface to the implementation; a property on each
switch keeps the panel's data behind the same permission as the switch
itself, with no new object.

Unit tests hold the 30-day window to acceptance test 11 (three days ago
listed, forty not), the hourly throttle and the keys. The e2e proves on a
running server that the admin's IMAP and submission sign-ins are listed
with their time, that a second sign-in within the hour isn't written again,
and that a tenant's list holds its own user and nobody outside the tenant.
All 70 checks pass.

tests/e2e/legacy_protocols.py

@@ -30,7 +30,9 @@ leaves every other domain alone, and stops client configuration offering
 legacy servers for those domains. It reaches only its own tenant's switch,
 and can't turn it back on while the server has legacy protocols off
 (acceptance tests 6 to 10, 14). Throughout, the JMAP session tells each
-account which way its switches point (test 13).
+account which way its switches point (test 13), and the impact panel's
+list names who signed in over what: every account at server scope, only the
+tenant's own at tenant scope, rewritten at most once an hour (LP-15).
 
 Passwords are generated into files under target/e2e and never printed.
 Everything is removed afterwards unless KEEP=1.
@@ -292,6 +294,14 @@ def tenant_checks(admin, admin_pw, account):
 
     check(session_flag(tu, user_pw) == "enabled",
           "the session says enabled for the tenant's user while both switches are on (test 13)")
+    imap_login(PORTS["imap"], tu, user_pw)
+    got = tget()
+    recent = got[1]["list"][0].get("recentLegacyUse", [])
+    names = {(r["name"], r["protocol"]) for r in recent}
+    check((tu, "imap") in names and not any(n == admin for n, _ in names),
+          "the tenant's panel lists its own user's IMAP sign-in and nobody outside it (LP-15, MT-1)")
+    if (tu, "imap") not in names:
+        print("     recent:", recent)
 
     # Before: the tenant's user signs in, and its domain is offered IMAP.
     check(imap_login(PORTS["imap"], tu, user_pw).startswith("OK"),
@@ -460,6 +470,22 @@ def main():
     check(smtp_auths(PORTS["submissions"], admin, [admin_pw])[0].startswith("235"),
           "submission sign-in works with the switch on")
 
+    # The impact panel (LP-15): the sign-ins above are on it, once each.
+    got = one(admin, admin_pw, "inbuxa:ProtocolPolicy/get",
+              {"accountId": account, "ids": None, "properties": ["recentLegacyUse"]})
+    recent = got[1]["list"][0].get("recentLegacyUse", [])
+    mine = {r["protocol"]: r for r in recent if r["name"] == admin}
+    check(set(mine) == {"imap", "submission"} and all(r["lastUsedAt"] > 0 for r in mine.values()),
+          "the panel lists the admin's IMAP and submission sign-ins, and when (LP-15)")
+    if set(mine) != {"imap", "submission"}:
+        print("     recent:", recent)
+    imap_login(PORTS["imap"], admin, admin_pw)
+    got = one(admin, admin_pw, "inbuxa:ProtocolPolicy/get",
+              {"accountId": account, "ids": None, "properties": ["recentLegacyUse"]})
+    again = {r["protocol"]: r for r in got[1]["list"][0].get("recentLegacyUse", []) if r["name"] == admin}
+    check(again.get("imap", {}).get("lastUsedAt") == mine.get("imap", {}).get("lastUsedAt"),
+          "a second sign-in within the hour isn't written again (LP-15)")
+
     # What the screen reads: the locked set and what would close (LP-16, LP-21).
     got = one(admin, admin_pw, "inbuxa:ProtocolPolicy/get", policy_get)
     if got[0] != "inbuxa:ProtocolPolicy/get":