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_files_full.py

@@ -94,8 +94,6 @@ class TestE2EFilesFull(unittest.TestCase):
                 self.repo_name,
                 "--source-volume",
                 self.volume_src,
-                "--rsync-image",
-                "ghcr.io/kevinveenbirkenbach/alpine-rsync",
             ]
         )