Add retry logic for Synapse user registration on Connection refused

When DB credentials change (derived from matrix_synapse_macaroon_secret_key),
a running Synapse container may fail to connect to its database and stop
serving requests. This causes register_new_matrix_user to fail with
"Connection refused" when the matrix-user-creator role tries to register users.

This extends the retry logic from 44b43a51b (which handled HMAC failures)
to also handle Connection refused errors: restart Synapse (picking up the
new config with updated credentials), wait for it to start, and retry.

Caused by c21a80d232

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

roles/custom/matrix-user-creator/tasks/util/ensure_user_registered_synapse.yml

@@ -31,10 +31,13 @@
     matrix_synapse_register_user_result.rc != 0
     and 'User ID already taken' not in matrix_synapse_register_user_result.stdout
     and 'HMAC incorrect' not in matrix_synapse_register_user_result.stdout
+    and 'Connection refused' not in matrix_synapse_register_user_result.stderr | default('')
 
-- when: "'HMAC incorrect' in matrix_synapse_register_user_result.stdout | default('')"
+- when: >-
+    'HMAC incorrect' in matrix_synapse_register_user_result.stdout | default('')
+    or 'Connection refused' in matrix_synapse_register_user_result.stderr | default('')
   block:
-    - name: Restart Synapse due to HMAC failure (likely a registration_shared_secret change)
+    - name: Restart Synapse due to registration failure (likely a registration_shared_secret or DB credentials change)
       ansible.builtin.service:
         name: "matrix-synapse.service"
         state: restarted