feat(slice-1): platform/db migration runner + migration 0001 (INV-7, §6.3 schema)

Forward-only, fail-stop, advisory-lock-guarded psycopg runner; migration 0001 lays
the account/auth_code/storefront/membership schema. Tests prove migrate-from-empty
and idempotent re-migrate (INV-1 partial) and the INV-4 many-capable shape.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/tests/conftest.py

@@ -0,0 +1,47 @@
+"""Test fixtures — a fresh, empty PostgreSQL database per test.
+
+Each test gets its own database created on the dev/CI Postgres (every test runs the
+real engine, §6.8). The admin DSN points at the `postgres` maintenance database;
+ECOMM_TEST_ADMIN_URL overrides it (CI sets it to the service container).
+"""
+from __future__ import annotations
+
+import os
+import uuid
+
+import psycopg
+import pytest
+
+ADMIN_URL = os.environ.get(
+    "ECOMM_TEST_ADMIN_URL", "postgresql://ecomm:ecomm@localhost:5432/postgres"
+)
+
+
+def _swap_dbname(dsn: str, dbname: str) -> str:
+    # ADMIN_URL ends in `/postgres`; point the same credentials at `dbname`.
+    base, _, _old = dsn.rpartition("/")
+    return f"{base}/{dbname}"
+
+
+@pytest.fixture()
+def fresh_db_url():
+    """Create a uniquely-named empty database; drop it after the test."""
+    dbname = f"ecomm_test_{uuid.uuid4().hex}"
+    admin = psycopg.connect(ADMIN_URL, autocommit=True)
+    try:
+        admin.execute(f'CREATE DATABASE "{dbname}"')
+    finally:
+        admin.close()
+    try:
+        yield _swap_dbname(ADMIN_URL, dbname)
+    finally:
+        admin = psycopg.connect(ADMIN_URL, autocommit=True)
+        try:
+            admin.execute(
+                "SELECT pg_terminate_backend(pid) FROM pg_stat_activity "
+                "WHERE datname = %s AND pid <> pg_backend_pid()",
+                (dbname,),
+            )
+            admin.execute(f'DROP DATABASE IF EXISTS "{dbname}"')
+        finally:
+            admin.close()