Move email types to pkg/email for cloud repo access

Extracts Mailer interface, EmailData, and Button to pkg/email/types.go
so the cloud repo can use them via ServerContext. internal/email re-exports
the types as aliases so existing callers are unchanged. Also fixes
pre-existing lint errors (unchecked rollback and deadline calls).

internal/api/handlers_me.go

@@ -533,7 +533,7 @@ func (h *meHandler) DeleteAccount(w http.ResponseWriter, r *http.Request) {
 		writeError(w, http.StatusInternalServerError, "db_error", "failed to start transaction")
 		return
 	}
-	defer tx.Rollback(ctx)
+	defer func() { _ = tx.Rollback(ctx) }()
 
 	qtx := h.db.WithTx(tx)