feat(gmail): the OAuth client can be entered instead of deployed

The one click that opens Google's consent screen already existed. What it needs
behind it is a registered client, and no app can conjure one: Google issues no
Gmail access to an unregistered client, and `gmail.readonly` is a restricted
scope, so an id shipped inside a public image would be both extractable from it
and unusable without Google's security review of every deployment using it.

The registration is a one-off. Having to edit a Rune's manifest and recreate the
container to paste the result was not.

`VERDANDE_GMAIL_CLIENT_ID` and `_SECRET` still win when set. A value in the
environment is a deliberate deployment decision, applied again on every recreate,
and a form that could quietly override it would make the manifest a lie — so the
page says where the value came from, and PUT answers 409.

The redirect URI is spelled out on the page rather than left to be reconstructed.
It is derived from VERDANDE_BASE_URL, it has to match Google's copy exactly, and
the error Google returns when it does not names neither value.

Its own table rather than `user_settings`: an OAuth client is the instance's, not
one person's — and `user_settings` has `user_id` as its whole primary key, so a
second scope there would silently overwrite the AI provider's key.

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

internal/httpapi/integrations_test.go

@@ -1153,3 +1153,130 @@ func TestMCPCreateRefusesASecondLevelOfSubtask(t *testing.T) {
 		t.Errorf("a sub-task of a sub-task was accepted: %v", response)
 	}
 }
+
+// The Gmail OAuth client, enterable rather than deployed.
+//
+// The registration itself cannot be avoided — Google issues no Gmail access to an
+// unregistered client — but having to edit a Rune's manifest and recreate the
+// container to paste the result can be, and that is what this covers.
+func TestTheGmailClientCanBeSetFromTheInterface(t *testing.T) {
+	ts := newTestServer(t)
+	ts.bootstrap(t)
+
+	_, empty := ts.do(t, "GET", "/api/v1/gmail/client", nil)
+	if empty["client_id"] != "" || empty["has_secret"] != false {
+		t.Errorf("a fresh instance already has a client: %v", empty)
+	}
+	if empty["from_env"] != false {
+		t.Errorf("from_env = %v with nothing in the environment", empty["from_env"])
+	}
+	// The single most likely thing to be wrong, so it is spelled out rather than
+	// left for somebody to reconstruct.
+	if empty["redirect_uri"] != "http://127.0.0.1/oauth/gmail/callback" {
+		t.Errorf("redirect_uri = %v", empty["redirect_uri"])
+	}
+
+	resp, _ := ts.do(t, "PUT", "/api/v1/gmail/client", map[string]any{
+		"client_id": "1234.apps.googleusercontent.com", "client_secret": "hemmelig",
+	})
+	if resp.StatusCode != http.StatusNoContent {
+		t.Fatalf("save: status %d", resp.StatusCode)
+	}
+
+	_, saved := ts.do(t, "GET", "/api/v1/gmail/client", nil)
+	if saved["client_id"] != "1234.apps.googleusercontent.com" {
+		t.Errorf("client_id = %v", saved["client_id"])
+	}
+	if saved["has_secret"] != true {
+		t.Error("has_secret is false after saving one")
+	}
+	// Never read back. A settings page that repopulates a password field is one
+	// that will eventually leak into a screenshot.
+	if _, present := saved["client_secret"]; present {
+		t.Error("the secret came back in the response")
+	}
+
+	// Saving only the id must not clear the secret — otherwise correcting a typo
+	// silently breaks the connection.
+	ts.do(t, "PUT", "/api/v1/gmail/client", map[string]any{
+		"client_id": "5678.apps.googleusercontent.com",
+	})
+	_, again := ts.do(t, "GET", "/api/v1/gmail/client", nil)
+	if again["client_id"] != "5678.apps.googleusercontent.com" || again["has_secret"] != true {
+		t.Errorf("saving the id alone lost the secret: %v", again)
+	}
+
+	// And the authorise call now has something to build a URL with, which is the
+	// whole point: one click, and Google's consent screen.
+	resp, body := ts.do(t, "POST", "/api/v1/gmail/authorize", nil)
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("authorize with a stored client: status %d, body %v", resp.StatusCode, body)
+	}
+	if url, _ := body["url"].(string); !strings.Contains(url, "5678.apps.googleusercontent.com") {
+		t.Errorf("the consent URL does not carry the stored client: %v", body["url"])
+	}
+}
+
+// The environment wins, and says so. A value in the Rune's manifest is applied on
+// every recreate, so a form that could quietly override it would make the manifest
+// a lie.
+func TestTheEnvironmentsGmailClientCannotBeOverwritten(t *testing.T) {
+	ts := newTestServerWith(t, func(c *config.Config) {
+		c.GmailClientID = "fra-miljøet.apps.googleusercontent.com"
+		c.GmailClientSecret = "også-fra-miljøet"
+	})
+	ts.bootstrap(t)
+
+	_, shown := ts.do(t, "GET", "/api/v1/gmail/client", nil)
+	if shown["from_env"] != true {
+		t.Errorf("from_env = %v with the environment set", shown["from_env"])
+	}
+	if shown["client_id"] != "fra-miljøet.apps.googleusercontent.com" {
+		t.Errorf("client_id = %v", shown["client_id"])
+	}
+
+	resp, _ := ts.do(t, "PUT", "/api/v1/gmail/client", map[string]any{
+		"client_id": "min-egen", "client_secret": "min-egen",
+	})
+	if resp.StatusCode != http.StatusConflict {
+		t.Fatalf("overwriting the environment: status %d, want 409", resp.StatusCode)
+	}
+
+	_, after := ts.do(t, "GET", "/api/v1/gmail/client", nil)
+	if after["client_id"] != "fra-miljøet.apps.googleusercontent.com" {
+		t.Errorf("the environment's client was replaced: %v", after["client_id"])
+	}
+}
+
+// Administrators only, and sessions only: this is the instance's identity to
+// Google, and a leaked token must not be able to read or replace it.
+func TestTheGmailClientIsAdminsOnlyAndSessionsOnly(t *testing.T) {
+	ts := newTestServer(t)
+	ts.bootstrap(t)
+	other := ts.newUser(t, "anden@example.dk", "Anden")
+
+	for _, c := range []struct {
+		method string
+		body   any
+	}{{"GET", nil}, {"PUT", map[string]any{"client_id": "x"}}} {
+		if resp, _ := other.do(t, c.method, "/api/v1/gmail/client", c.body); resp.StatusCode != http.StatusForbidden {
+			t.Errorf("%s as an ordinary user: status %d, want 403", c.method, resp.StatusCode)
+		}
+	}
+
+	admin, err := ts.db.UserByEmail(t.Context(), "kristian@example.dk")
+	if err != nil {
+		t.Fatal(err)
+	}
+	token := ts.apiToken(t, admin.ID)
+	req, _ := http.NewRequest("GET", ts.URL+"/api/v1/gmail/client", nil)
+	req.Header.Set("Authorization", "Bearer "+token)
+	resp, err := (&http.Client{}).Do(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	resp.Body.Close()
+	if resp.StatusCode != http.StatusForbidden {
+		t.Errorf("with an admin's API token: status %d, want 403", resp.StatusCode)
+	}
+}