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()

backend/tests/test_migrations.py

@@ -0,0 +1,43 @@
+import psycopg
+
+from app.platform import db
+
+_TABLES = {"account", "auth_code", "storefront", "storefront_membership"}
+
+
+def _table_names(conn) -> set[str]:
+    rows = conn.execute(
+        "SELECT tablename FROM pg_tables WHERE schemaname = 'public'"
+    ).fetchall()
+    return {r[0] for r in rows}
+
+
+def test_migrate_from_empty_applies_0001(fresh_db_url):
+    with psycopg.connect(fresh_db_url) as conn:
+        applied = db.migrate(conn)
+    assert applied == ["0001_init.sql"]
+    with psycopg.connect(fresh_db_url) as conn:
+        assert _TABLES.issubset(_table_names(conn))
+
+
+def test_migrate_is_idempotent(fresh_db_url):
+    with psycopg.connect(fresh_db_url) as conn:
+        db.migrate(conn)
+    with psycopg.connect(fresh_db_url) as conn:
+        applied_again = db.migrate(conn)
+    assert applied_again == []
+
+
+def test_membership_has_no_unique_account_constraint(fresh_db_url):
+    # INV-4: the schema must keep many-storefronts-per-account open — no UNIQUE on
+    # storefront_membership.account_id anywhere.
+    with psycopg.connect(fresh_db_url) as conn:
+        db.migrate(conn)
+        rows = conn.execute(
+            """
+            SELECT indexdef FROM pg_indexes
+            WHERE schemaname = 'public' AND tablename = 'storefront_membership'
+            """
+        ).fetchall()
+    defs = " ".join(r[0] for r in rows).lower()
+    assert "unique" not in defs.replace("primary key", "") or "(account_id)" not in defs