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/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
 );