fix(backup)!: read the instance from one engine-name set, and trust it

Two shapes fell through the inline regex, which knew `database`, `db` and
`postgres` only. A container named exactly after its engine - what a compose
file writes as `container_name: postgres` - carries no separator before the
token, so it resolved to nothing and 6.0.0 stopped dumping it without saying
so. And a swarm task of a central MariaDB reads `mariadb_mariadb.1.<id>`,
where `_mariadb` was no token at all, so that database has never been dumped
under swarm at all.

ENGINE_NAMES states the set once and serves both readings: carried as a
suffix it makes the rest the instance, being one outright makes the container
its own instance.

backup_mariadb_or_postgres stops calling an application container a database.
container_engine recognises an engine by its client tools, which an
application image often ships, so refusing the dump alone would have recorded
the volume as `database: true, dumped: false` - the exact shape a restore
drill reads as a database that was missed. Without an instance there is no
database to record.

BREAKING CHANGE: `mariadb` and `mysql` join the suffix tokens, so a container
named `<app>-mariadb` resolves to the instance `<app>` rather than to its own
name. A databases.csv keyed on the full container name has to move to the
application name, or name the container in --database-containers.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/unit/backup/test_get_instance.py

@@ -24,12 +24,21 @@ class TestDeclaredContainers(unittest.TestCase):
             get_instance("shop-database", ["shop-database"]), "shop-database"
         )
 
-    def test_an_undeclared_central_engine_resolves_to_nothing(self) -> None:
-        """`postgres-central` has no separator before its token, so nothing is
-        stripped - a central engine has to be declared to be found."""
+    def test_a_qualified_central_name_still_has_to_be_declared(self) -> None:
         self.assertIsNone(get_instance("postgres-central", []))
 
 
+class TestContainersNamedAfterTheirEngine(unittest.TestCase):
+    def test_a_bare_engine_name_is_its_own_instance(self) -> None:
+        for name in ("postgres", "mariadb", "mysql", "db", "database"):
+            with self.subTest(container=name):
+                self.assertEqual(get_instance(name, []), name)
+
+    def test_a_swarm_task_of_such_a_container_keeps_the_instance(self) -> None:
+        self.assertEqual(get_instance("postgres_postgres.1.k3f9x2", []), "postgres")
+        self.assertEqual(get_instance("mariadb_mariadb.1.k3f9x2", []), "mariadb")
+
+
 class TestDedicatedEngines(unittest.TestCase):
     def test_compose_names_the_container_with_a_hyphen(self) -> None:
         self.assertEqual(get_instance("discourse-database", []), "discourse")
@@ -49,6 +58,10 @@ class TestDedicatedEngines(unittest.TestCase):
     def test_mariadb_uses_the_same_suffix(self) -> None:
         self.assertEqual(get_instance("matomo-database", []), "matomo")
 
+    def test_an_engine_named_suffix_is_stripped_too(self) -> None:
+        self.assertEqual(get_instance("shop-mariadb", []), "shop")
+        self.assertEqual(get_instance("shop-mysql", []), "shop")
+
 
 class TestApplicationContainers(unittest.TestCase):
     def test_a_bare_application_name_is_not_a_database(self) -> None: