feat(auth): see where you are signed in, and end one of them

`last_seen_at` has been written on every request since sessions existed —
throttled to once a minute, deliberately, so a list could one day say
"denne enhed, lige nu" — and nothing has ever read it. The store even had
ListSessions. What was missing was two routes and a panel.

GET /auth/sessions lists them, most recently used first, with the current
one marked so nobody has to work out which row is the browser they are
reading it in. DELETE /auth/sessions/{id} ends one. Ending the current
session is allowed and is simply logging out: refusing it would be a rule
with no reason behind it, and somebody clearing every device wants this
one gone too.

Both are behind requireSession rather than the ambient auth, the same as
the API tokens. A leaked token that could list its owner's devices and end
their sessions turns a theft into a lockout.

The delete is scoped in the statement rather than checked and then run.
An id read in one step and deleted in the next is a window, and the whole
reason somebody is on this screen is that they think another person is in
their account.

The device names are a handful of substring tests, not a parsing library.
The question this answers is "is one of these me?", and for that a wrong
guess between two Chromium forks costs nothing while a dependency that has
to be kept current costs something every year. Order is the whole
correctness of it: an iPhone's user agent says Mac OS X, Edge says Chrome,
and Chrome says Safari. The raw string is sent alongside and sits in the
row's title, so anything the summary gets wrong is still visible — and
something that is not a browser at all shows its own name rather than
"Ukendt", because a CalDAV client is a real session.

Times are relative. The question is "was that me, just now?", and an
absolute timestamp makes the reader do the subtraction; past a week it
switches to a date, because "for 23 dage siden" is a number nobody can
place.

The smoke test signs in a second time in a second browser context, ends
that session from the first, and checks the other browser is really signed
out — and that this one is not.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/httpapi/auth_test.go

@@ -761,3 +761,137 @@ func TestUnknownSessionTokenIsUnauthorized(t *testing.T) {
 		t.Errorf("status %d, want 401", resp.StatusCode)
 	}
 }
+
+// last_seen_at has been written on every request since sessions existed, for
+// exactly this list, and nothing read it. A session you cannot see is a session
+// you cannot end.
+func TestSessionsCanBeListedAndEnded(t *testing.T) {
+	ts := newTestServer(t)
+	ts.bootstrap(t)
+
+	// A second sign-in from "another device": same account, its own cookie jar.
+	other := &testServer{Server: ts.Server, db: ts.db, client: newJarClient(t)}
+	resp, _ := other.do(t, "POST", "/api/v1/auth/login", map[string]string{
+		"email": "kristian@example.dk", "password": "et langt kodeord",
+	})
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("second login: status %d", resp.StatusCode)
+	}
+
+	resp, body := ts.do(t, "GET", "/api/v1/auth/sessions", nil)
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("list sessions: %d %v", resp.StatusCode, body)
+	}
+	sessions, _ := body["sessions"].([]any)
+	if len(sessions) != 2 {
+		t.Fatalf("want two sessions, got %v", sessions)
+	}
+
+	// Exactly one is this one, or the interface cannot tell somebody which row is
+	// the browser they are reading it in.
+	current := ""
+	for _, raw := range sessions {
+		s := raw.(map[string]any)
+		if s["current"] == true {
+			if current != "" {
+				t.Fatal("two sessions both claim to be the current one")
+			}
+			current = s["id"].(string)
+		}
+		if s["device"] == "" {
+			t.Errorf("no device summary: %v", s)
+		}
+		if s["last_seen_at"] == nil {
+			t.Errorf("no last_seen_at, which is the whole reason the column is written: %v", s)
+		}
+	}
+	if current == "" {
+		t.Fatal("none of the sessions is marked as the current one")
+	}
+
+	// End the other one.
+	var otherID string
+	for _, raw := range sessions {
+		if s := raw.(map[string]any); s["id"] != current {
+			otherID = s["id"].(string)
+		}
+	}
+	if resp, _ := ts.do(t, "DELETE", "/api/v1/auth/sessions/"+otherID, nil); resp.StatusCode != http.StatusNoContent {
+		t.Fatalf("delete session: status %d", resp.StatusCode)
+	}
+
+	// That device is signed out, and this one is not.
+	if r, _ := other.do(t, "GET", "/api/v1/auth/me", nil); r.StatusCode != http.StatusUnauthorized {
+		t.Errorf("the ended session still works: status %d", r.StatusCode)
+	}
+	if r, _ := ts.do(t, "GET", "/api/v1/auth/me", nil); r.StatusCode != http.StatusOK {
+		t.Errorf("ending another session logged this one out: status %d", r.StatusCode)
+	}
+}
+
+// Somebody else's session is not yours to end, even with its id in hand.
+func TestASessionCanOnlyBeEndedByItsOwner(t *testing.T) {
+	ts := newTestServer(t)
+	ts.bootstrap(t)
+	other := ts.newUser(t, "anden@example.dk", "Anden")
+
+	_, body := ts.do(t, "GET", "/api/v1/auth/sessions", nil)
+	mine := body["sessions"].([]any)[0].(map[string]any)["id"].(string)
+
+	if resp, _ := other.do(t, "DELETE", "/api/v1/auth/sessions/"+mine, nil); resp.StatusCode != http.StatusNotFound {
+		t.Errorf("status %d, want 404", resp.StatusCode)
+	}
+	// And it still works.
+	if r, _ := ts.do(t, "GET", "/api/v1/auth/me", nil); r.StatusCode != http.StatusOK {
+		t.Errorf("somebody else ended my session: status %d", r.StatusCode)
+	}
+}
+
+// An API token is accepted almost everywhere, and must not be accepted here: a
+// leaked token that could end its owner's sessions, or even list their devices,
+// turns a theft into a lockout.
+func TestSessionsAreNotReachableWithAToken(t *testing.T) {
+	ts := newTestServer(t)
+	ts.bootstrap(t)
+
+	user, err := ts.db.UserByEmail(t.Context(), "kristian@example.dk")
+	if err != nil {
+		t.Fatal(err)
+	}
+	token := ts.apiToken(t, user.ID)
+
+	req, _ := http.NewRequest("GET", ts.URL+"/api/v1/auth/sessions", nil)
+	req.Header.Set("Authorization", "Bearer "+token)
+	resp, err := (&http.Client{}).Do(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusForbidden {
+		t.Errorf("status %d, want 403", resp.StatusCode)
+	}
+}
+
+func TestUserAgentsAreSummarisedForAPerson(t *testing.T) {
+	cases := []struct {
+		ua   string
+		want string
+	}{
+		{"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/131.0 Safari/537.36", "Chrome på macOS"},
+		// An iPhone's user agent says Mac OS X too, so the order of the tests is
+		// the whole correctness of this function.
+		{"Mozilla/5.0 (iPhone; CPU iPhone OS 17_0 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.0 Mobile/15E148 Safari/604.1", "Safari på iPhone"},
+		// And Edge claims to be Chrome, which claims to be Safari.
+		{"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 Chrome/131.0 Safari/537.36 Edg/131.0", "Edge på Windows"},
+		{"Mozilla/5.0 (X11; Linux x86_64; rv:133.0) Gecko/20100101 Firefox/133.0", "Firefox på Linux"},
+		{"", "Ukendt enhed"},
+		// Not a browser at all. Its own name beats a word that says the server
+		// gave up: a CalDAV client is a real session.
+		{"curl/8.4.0", "curl/8.4.0"},
+	}
+	for _, c := range cases {
+		if got := describeUserAgent(c.ua); got != c.want {
+			t.Errorf("describeUserAgent(%q) = %q, want %q", c.ua, got, c.want)
+		}
+	}
+}