Implement PostgreSQL production store path

Add the PostgreSQL backend, migration and stopped-write transfer tools, lease-aware deployment manifests, tenancy declarations, and shared conformance coverage. Persist grouping mutations in durable stores and separate process liveness from database readiness.

tests/test_backend_selection.py

@@ -0,0 +1,33 @@
+from dataclasses import replace
+
+import pytest
+
+from tenant_engine.config import Settings
+from tenant_engine.main import _build_store
+from tenant_engine.sqlite_store import SQLiteTenantStore
+
+
+def _settings() -> Settings:
+    return Settings(
+        flex_auth_base_url=None,
+        flex_auth_timeout_seconds=1,
+        host="127.0.0.1",
+        port=8090,
+    )
+
+
+def test_store_selection_defaults_to_memory_and_selects_sqlite(tmp_path) -> None:
+    assert _build_store(_settings()) is None
+    store = _build_store(replace(_settings(), database_path=str(tmp_path / "tenant.db")))
+    assert isinstance(store, SQLiteTenantStore)
+
+
+def test_store_selection_refuses_ambiguous_configuration(tmp_path) -> None:
+    with pytest.raises(RuntimeError, match="ambiguous store configuration"):
+        _build_store(
+            replace(
+                _settings(),
+                database_path=str(tmp_path / "tenant.db"),
+                database_url_file=str(tmp_path / "database-url"),
+            )
+        )