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/postgres_backend.py

@@ -0,0 +1,30 @@
+from __future__ import annotations
+
+import os
+from pathlib import Path
+
+import pytest
+
+from tenant_engine.postgres_store import PostgresTenantStore
+
+
+def clean_postgres_store(tmp_path: Path) -> PostgresTenantStore:
+    dsn = os.getenv("TENANT_ENGINE_TEST_DATABASE_URL", "").strip()
+    if not dsn:
+        pytest.skip("set TENANT_ENGINE_TEST_DATABASE_URL to exercise PostgreSQL conformance")
+    try:
+        import psycopg
+    except ImportError:
+        pytest.skip("install tenant-engine[postgres] to exercise PostgreSQL conformance")
+
+    migration = Path(__file__).parents[1] / "migrations/postgres/0001_tenant_store.sql"
+    with psycopg.connect(dsn, autocommit=True) as connection:
+        connection.execute(migration.read_text(encoding="utf-8"))
+        connection.execute(
+            """TRUNCATE guardrail_changes, guardrail_overrides,
+               idempotency_receipts, events, plans, grants, tenants
+               RESTART IDENTITY CASCADE"""
+        )
+    url_file = tmp_path / "postgres-url"
+    url_file.write_text(dsn, encoding="utf-8")
+    return PostgresTenantStore(str(url_file), min_pool_size=1, max_pool_size=4)