feat(gmail,update): finish the OAuth2 flow, and tell admins about new releases

Gmail now connects. Authorisation-code flow with PKCE, the verifier kept
server-side rather than in a cookie — the callback is a top-level
navigation from Google, and a SameSite=Lax cookie is the kind of thing
that works until a browser tightens its defaults. State is checked, the
attempt expires after ten minutes, and the spent verifier is deleted
rather than left at rest.

Scope is gmail.readonly and the sync is one-way on purpose. Unstarring a
message does not delete the task; completing the task does not touch the
mailbox. Two-way would mean deciding what happens when both sides
changed, and the only honest answer is a sync model far larger than the
feature. The query carries newer_than:30d so connecting an account with
a decade of starred mail does not create a decade of tasks.

The update check is off unless the operator turns it on, asks GitHub at
most every six hours, and sends nothing about the instance — no
identifier, no version, no counts. A development build never reports an
update: somebody running their own tree is ahead of every release, and
telling them otherwise trains them to ignore the notice. Only
administrators see it, because only they can act on it.

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

internal/store/notifications.go

@@ -270,6 +270,44 @@ func (db *DB) UserByMailToken(ctx context.Context, token string) (*User, error)
 	return db.scanUser(ctx, `SELECT `+userColumns+` FROM users WHERE mail_token = ?`, token)
 }
 
+// UsersWithGmail returns everybody who has connected a mailbox, which is who the
+// poller has to visit. Selected by the presence of a refresh token rather than by a
+// flag, so a disconnected account drops out of the sweep the moment its tokens are
+// cleared.
+func (db *DB) UsersWithGmail(ctx context.Context) ([]User, error) {
+	rows, err := db.QueryContext(ctx, `
+		SELECT u.id FROM users u
+		JOIN user_settings s ON s.user_id = u.id
+		WHERE s.scope = 'gmail' AND s.values_json LIKE '%"refresh_token":"%'`)
+	if err != nil {
+		return nil, err
+	}
+	var ids []string
+	for rows.Next() {
+		var id string
+		if err := rows.Scan(&id); err != nil {
+			rows.Close()
+			return nil, err
+		}
+		ids = append(ids, id)
+	}
+	err = rows.Err()
+	rows.Close()
+	if err != nil {
+		return nil, err
+	}
+
+	out := make([]User, 0, len(ids))
+	for _, id := range ids {
+		user, err := db.UserByID(ctx, id)
+		if err != nil {
+			continue
+		}
+		out = append(out, *user)
+	}
+	return out, nil
+}
+
 // --- per-user settings ------------------------------------------------------------------
 
 // UserSettings reads one scope of a person's integration settings. A missing row is