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_postgres_no_copy.py

@@ -2,6 +2,8 @@
 import unittest
 
 from .helpers import (
+    POSTGRES_IMAGE,
+    POSTGRES_DATA_DIR,
     backup_run,
     backup_path,
     cleanup_docker,
@@ -46,8 +48,8 @@ class TestE2EPostgresNoCopy(unittest.TestCase):
                 "-e",
                 "POSTGRES_USER=postgres",
                 "-v",
-                f"{cls.pg_volume}:/var/lib/postgresql/data",
-                "postgres:16",
+                f"{cls.pg_volume}:{POSTGRES_DATA_DIR}",
+                POSTGRES_IMAGE,
             ]
         )
         wait_for_postgres(cls.pg_container, user="postgres", timeout_s=90)