feat(gmail): take the star off a mail once it has become a task

A starred Gmail message became a task and kept its star, so a starred inbox only
grew — the IMAP side has always unflagged what it imports, and Gmail could not,
because the scope was gmail.readonly. This gives Gmail the same one write: after
the task is made, the STARRED label comes off the message, in one batchModify for
the whole sweep. Nothing else is touched — not the body, not other labels, and a
label trigger's label stays where it is, since that label is the person's own.

The scope goes from gmail.readonly to gmail.modify, which is the narrowest scope
Google offers that can remove a label. An existing connection keeps working on its
old read-only grant — the sync still reads, the unstar is a best-effort 403 that is
logged, not fatal — and starts clearing stars once the account is reconnected and
the new grant is in place. It self-heals: a mail that could not be unstarred stays
starred, stays in the next query, and is tried again.

The consent screen now asks for more than it used to; the settings copy, the docs
and the scope table all say modify rather than read-only in the same change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/gmail/gmail.go

@@ -1,14 +1,20 @@
 // Package gmail is the small slice of the Gmail API verdande needs, and the scope
 // it asks for.
 //
-// The connection is one-way on purpose: a starred or labelled message becomes a
-// task, and nothing verdande does afterwards touches the mailbox. Unstarring the
-// message does not delete the task, completing the task does not unstar the
-// message. Two-way would mean deciding what to do when both sides changed, and the
-// only honest answer to that is a synchronisation model far bigger than the feature.
+// The one thing verdande writes back is the star. A starred message becomes a
+// task, and once it has, the star comes off — exactly as the IMAP side takes the
+// flag off a mail it has imported, and for the same reason: the star meant "turn
+// this into a task", the task now exists, so the star is no longer true. Nothing
+// else is touched. Completing the task does not reach back to the mail; the mail
+// is a pointer the task was made from, not a copy the two have to be kept in step.
 //
-// Scope is gmail.readonly. verdande never needs to send, modify or delete, and
-// asking for less is what makes the consent screen something a person can agree to.
+// That one write is why the scope is gmail.modify rather than gmail.readonly.
+// Removing the STARRED label is a modify, and Gmail offers no narrower scope that
+// can do it — so the consent screen asks for more than it used to, and the trade
+// is deliberate: a starred inbox that empties itself as the tasks are made, rather
+// than one that fills with stars nothing ever clears. Still not send, still not
+// delete — modify is labels, and the only label verdande removes is the one that
+// triggered the task.
 //
 // The OAuth flow itself moved to internal/google when Calendar needed the same one.
 // The names below are aliases so that every call site — and this package's own
@@ -27,8 +33,10 @@ import (
 	"github.com/kristianwind/verdande/internal/google"
 )
 
-// Read-only, and only mail. Not profile, not contacts, not send.
-const Scope = "https://www.googleapis.com/auth/gmail.readonly"
+// Read mail and change its labels, and only mail. Not profile, not contacts, not
+// send, not delete. The single write is removing the STARRED label from a message
+// that has become a task; see the package comment for why that costs modify.
+const Scope = "https://www.googleapis.com/auth/gmail.modify"
 
 const defaultAPIBase = "https://gmail.googleapis.com/gmail/v1"
 
@@ -169,6 +177,25 @@ func (c *Client) get(ctx context.Context, url string, out any) error {
 	return google.Get(ctx, c.http, c.accessToken, url, out)
 }
 
+// Unstar takes the star off the given messages — one call for all of them, the way
+// the IMAP side unflags a batch, so a sweep that made ten tasks is one write and
+// not ten.
+//
+// STARRED is a system label whose id is its name, so no lookup is needed; a custom
+// label would need one, and verdande removes none. batchModify answers 204 with no
+// body on success. An empty list is a no-op, not a call: batchModify with no ids is
+// an error on Google's side and nothing worth making a round trip to hear.
+func (c *Client) Unstar(ctx context.Context, ids []string) error {
+	if len(ids) == 0 {
+		return nil
+	}
+	body := struct {
+		IDs            []string `json:"ids"`
+		RemoveLabelIDs []string `json:"removeLabelIds"`
+	}{IDs: ids, RemoveLabelIDs: []string{"STARRED"}}
+	return google.Post(ctx, c.http, c.accessToken, c.api()+"/users/me/messages/batchModify", body, nil)
+}
+
 // SenderName pulls "Anders Jensen" out of "Anders Jensen <anders@example.dk>",
 // falling back to the address when there is no display name.
 func SenderName(from string) string {

internal/gmail/gmail_test.go

@@ -1,16 +1,81 @@
 package gmail
 
 import (
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
 	"strings"
 	"testing"
 )
 
 // The scope is the one thing verdande chooses about the connection, and it is the
 // one the person reads on the consent screen. The flow itself is exercised in
-// internal/google; what belongs here is that Gmail still asks for read-only mail.
-func TestTheScopeIsReadOnlyMail(t *testing.T) {
-	if !strings.HasSuffix(Scope, "gmail.readonly") {
-		t.Errorf("scope = %q; verdande never needs to send or modify", Scope)
+// internal/google; what belongs here is that Gmail asks for modify — read plus the
+// one write it makes, taking the star off an imported message — and nothing wider.
+func TestTheScopeIsModifyMail(t *testing.T) {
+	if !strings.HasSuffix(Scope, "gmail.modify") {
+		t.Errorf("scope = %q; want gmail.modify — read, plus removing the star", Scope)
+	}
+	// Modify is the ceiling. Send, or the full account, would be a different promise
+	// to the person on the consent screen than the one this feature needs.
+	for _, wider := range []string{"gmail.send", "mail.google.com", "gmail.settings"} {
+		if strings.Contains(Scope, wider) {
+			t.Errorf("scope = %q; must not include %q", Scope, wider)
+		}
+	}
+}
+
+// Unstar removes the star from a batch in one call, and it addresses the messages
+// it was given and only the STARRED label.
+func TestUnstarRemovesOnlyTheStar(t *testing.T) {
+	var gotPath string
+	var gotBody struct {
+		IDs            []string `json:"ids"`
+		AddLabelIDs    []string `json:"addLabelIds"`
+		RemoveLabelIDs []string `json:"removeLabelIds"`
+	}
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotPath = r.URL.Path
+		raw, _ := io.ReadAll(r.Body)
+		_ = json.Unmarshal(raw, &gotBody)
+		w.WriteHeader(http.StatusNoContent) // what batchModify answers on success
+	}))
+	defer srv.Close()
+
+	err := NewClient("token").At(srv.URL).Unstar(context.Background(), []string{"a", "b"})
+	if err != nil {
+		t.Fatalf("Unstar: %v", err)
+	}
+	if !strings.HasSuffix(gotPath, "/users/me/messages/batchModify") {
+		t.Errorf("path = %q, want the batchModify endpoint", gotPath)
+	}
+	if strings.Join(gotBody.IDs, ",") != "a,b" {
+		t.Errorf("ids = %v, want [a b]", gotBody.IDs)
+	}
+	if strings.Join(gotBody.RemoveLabelIDs, ",") != "STARRED" {
+		t.Errorf("removeLabelIds = %v, want [STARRED]", gotBody.RemoveLabelIDs)
+	}
+	if len(gotBody.AddLabelIDs) != 0 {
+		t.Errorf("addLabelIds = %v, want none — verdande adds no label", gotBody.AddLabelIDs)
+	}
+}
+
+// An empty batch is not a request: Gmail rejects a batchModify with no ids, and
+// there is nothing to say to it anyway.
+func TestUnstarOfNothingMakesNoCall(t *testing.T) {
+	called := false
+	srv := httptest.NewServer(http.HandlerFunc(func(http.ResponseWriter, *http.Request) {
+		called = true
+	}))
+	defer srv.Close()
+
+	if err := NewClient("token").At(srv.URL).Unstar(context.Background(), nil); err != nil {
+		t.Fatalf("Unstar(nil): %v", err)
+	}
+	if called {
+		t.Error("Unstar(nil) made a request; it should make none")
 	}
 }