Add XAP registrar opt-in database schema (#3134)

This commit implements the first stage of the Two-PR database schema
deployment for registrar opt-in to the Expiry Access Period (XAP), along
with updating GEMINI.md to mandate strict adherence to db/README.md for all
future database schema modifications.

Specifically, it:
- Adds Flyway migration
  V224__add_registrar_expiry_access_period_enabled.sql, which adds the
  expiry_access_period_enabled boolean column (DEFAULT false NOT NULL) to
  the Registrar table.
- Updates flyway.txt, nomulus.golden.sql, and ER diagrams to reflect the
  schema changes.
- Updates GEMINI.md with explicit instructions for AI agents to always
  consult db/README.md, follow the mandatory Two-PR deployment split when
  altering database schemas, and run both :db:test and
  :core:sqlIntegrationTest whenever database schema or ORM mappings are
  touched.
- Updates golden screenshot
  ConsoleScreenshotTest_globalRole_registrars_registrarsPage.png to reflect
  the new default database ordering after adding the column. (Note: A
  separate pull request, PR #3136 / http://go/r3pr/3136, permanently fixes
  this underlying sort non-determinism across our persistence layer by
  enforcing deterministic alphabetical ordering in Registrar.loadAllSorted).

TAG=agy
CONV=f2488c74-8b4a-43f1-9c22-d1dddbdbb4e0
BUG=http://b/437398822

db/src/main/resources/sql/er_diagram/brief_er_diagram.html

@@ -261,11 +261,11 @@ td.section {
     </tr>
     <tr>
      <td class="property_name">generated on</td>
-     <td class="property_value">2026-06-23 01:40:35</td>
+     <td class="property_value">2026-07-06 17:57:29</td>
     </tr>
     <tr>
      <td class="property_name">last flyway file</td>
-     <td id="lastFlywayFile" class="property_value">V223__tld_change_xap_enabled_to_transitions.sql</td>
+     <td id="lastFlywayFile" class="property_value">V224__add_registrar_expiry_access_period_enabled.sql</td>
     </tr>
    </tbody>
   </table>
@@ -273,7 +273,7 @@ td.section {
   <p>&nbsp;</p>
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db/src/main/resources/sql/flyway.txt

@@ -221,3 +221,4 @@ V220__domain_package_token_idx.sql
 V221__remove_contact_history.sql
 V222__remove_contact.sql
 V223__tld_change_xap_enabled_to_transitions.sql
+V224__add_registrar_expiry_access_period_enabled.sql

db/src/main/resources/sql/flyway/V224__add_registrar_expiry_access_period_enabled.sql

@@ -0,0 +1,20 @@
+-- Copyright 2026 The Nomulus Authors. All Rights Reserved.
+--
+-- Licensed under the Apache License, Version 2.0 (the "License");
+-- you may not use this file except in compliance with the License.
+-- You may obtain a copy of the License at
+--
+--     http://www.apache.org/licenses/LICENSE-2.0
+--
+-- Unless required by applicable law or agreed to in writing, software
+-- distributed under the License is distributed on an "AS IS" BASIS,
+-- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+-- See the License for the specific language governing permissions and
+-- limitations under the License.
+
+-- Add the XAP opt-in column to Registrar, defaulting to false for all existing registrars.
+-- To ensure backward compatibility with running servers (old Java code) during
+-- the transition phase of the deployment, we set DEFAULT false NOT NULL.
+-- TODO(mcilwain): Drop this DEFAULT constraint in a subsequent schema release once the Java code has been fully deployed.
+ALTER TABLE "Registrar" ADD COLUMN expiry_access_period_enabled boolean
+    DEFAULT false NOT NULL;

db/src/main/resources/sql/schema/nomulus.golden.sql

@@ -855,7 +855,8 @@ CREATE TABLE public."Registrar" (
     whois_server text,
     last_expiring_cert_notification_sent_date timestamp with time zone,
     last_expiring_failover_cert_notification_sent_date timestamp with time zone,
-    last_poc_verification_date timestamp with time zone
+    last_poc_verification_date timestamp with time zone,
+    expiry_access_period_enabled boolean DEFAULT false NOT NULL
 );