Add pgAdmin to the peo4c image

Install pgAdmin 9.18 in its own virtualenv and run it under gunicorn on
port 5050 next to PostgreSQL and the bridge. On first start the
entrypoint creates the pgAdmin login (PGADMIN_DEFAULT_EMAIL, default
admin@example.com; PGADMIN_DEFAULT_PASSWORD, default POSTGRES_PASSWORD)
and registers the bundled PostgreSQL server. pgAdmin is optional
(PEO4C_PGADMIN=false) and a failure in it does not stop the container.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docker/pgadmin/config_distro.py

@@ -0,0 +1,18 @@
+# pgAdmin settings for the peo4c image, loaded via CONFIG_DISTRO_FILE_PATH.
+# Paths derived from DATA_DIR in pgAdmin's config.py are computed before this
+# file is applied, so they are all set explicitly here.
+
+DATA_DIR = '/var/lib/pgadmin'
+SQLITE_PATH = DATA_DIR + '/pgadmin4.db'
+SESSION_DB_PATH = DATA_DIR + '/sessions'
+STORAGE_DIR = DATA_DIR + '/storage'
+KERBEROS_CCACHE_DIR = DATA_DIR + '/krbccache'
+AZURE_CREDENTIAL_CACHE_DIR = DATA_DIR + '/azurecredentialcache'
+LOG_FILE = '/dev/null'
+
+SERVER_MODE = True
+UPGRADE_CHECK_ENABLED = False
+
+# From the config_distro.py shipped in the pgadmin4 wheel, which this replaces.
+HELP_PATH = '../../docs/en_US/_build/html/'
+MINIFY_HTML = False