test(e2e): SPOT db images, PG18 mount layout, drop removed flags

Add a single source of truth in tests/e2e/helpers.py for the database
images and their in-container data dirs (POSTGRES_IMAGE=postgres:alpine,
POSTGRES_DATA_DIR=/var/lib/postgresql, MARIADB_IMAGE=mariadb:latest,
MARIADB_DATA_DIR=/var/lib/mysql) and route every test through it.
postgres:alpine now tracks 18+, which refuses a mount at
/var/lib/postgresql/data and stores data under /var/lib/postgresql, so
the mounts and the marker path move there. Drop the --rsync-image and the
renamed hard-restart flag from the invocations.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/e2e/test_e2e_mariadb_anonymous_preemption.py

@@ -30,6 +30,8 @@ import pandas
 from baudolo.backup import db as db_mod
 
 from .helpers import (
+    MARIADB_IMAGE,
+    MARIADB_DATA_DIR,
     cleanup_docker,
     require_docker,
     run,
@@ -69,8 +71,8 @@ class TestE2EMariaDBAnonymousPreemption(unittest.TestCase):
                 "-e",
                 f"MARIADB_ROOT_PASSWORD={cls.root_password}",
                 "-v",
-                f"{cls.db_volume}:/var/lib/mysql",
-                "mariadb:12.2",
+                f"{cls.db_volume}:{MARIADB_DATA_DIR}",
+                MARIADB_IMAGE,
             ]
         )