docs: describe create_savepoint session instead of removed CustomAsyncSession

AGENTS.md

@@ -27,9 +27,9 @@ Python is 3.14, dependencies managed by `uv`. The API is exposed on `:8000`.
 - `session` — request-scoped, bound to `dynamic_engine`, finalizer closes the session
 - `*_repository` — request-scoped, depend on `session`, configured with `auto_commit=True` (advanced-alchemy commits on success / rolls back on exception)
 
-**Persistence**: Models inherit `advanced_alchemy.base.BigIntAuditBase` (gives `id: BigInt`, `created_at`, `updated_at`). Metadata is shared with `orm.DeclarativeBase.metadata` in `app/models.py` so Alembic autogen sees everything. Repositories are `SQLAlchemyAsyncRepositoryService[Model]` with a nested `BaseRepository(SQLAlchemyAsyncRepository[Model])`. Custom `CustomAsyncSession` in `app/resources/db.py` overrides `close()` so test transactions are not actually closed when the session is bound to an `AsyncConnection` — this is what makes the per-test rollback fixture work.
+**Persistence**: Models inherit `advanced_alchemy.base.BigIntAuditBase` (gives `id: BigInt`, `created_at`, `updated_at`). Metadata is shared with `orm.DeclarativeBase.metadata` in `app/models.py` so Alembic autogen sees everything. Repositories are `SQLAlchemyAsyncRepositoryService[Model]` with a nested `BaseRepository(SQLAlchemyAsyncRepository[Model])`. `create_session` in `app/resources/db.py` builds a plain `AsyncSession` with `join_transaction_mode="create_savepoint"`. This is inert in production (the session binds to an engine) but enables the per-test rollback fixture: when a test binds the session to a connection already in a transaction, the session owns its own savepoint so the outer transaction survives commits. Do not "fix" it.
 
-**Test isolation** (`tests/conftest.py`): `db_session` fixture opens a connection, starts a transaction, starts a SAVEPOINT, then **overrides** `Dependencies.dynamic_engine` in the DI container to return that connection. All requests in the test reuse this connection; the outer transaction is rolled back in teardown, so DB state is clean between tests with no truncation needed. `app` and `client` fixtures build the real app and run it through `httpx.ASGITransport` + `asgi_lifespan.LifespanManager`. Polyfactory `SQLAlchemyFactory` is wired up via `set_async_session_in_base_sqlalchemy_factory`.
+**Test isolation** (`tests/conftest.py`): `db_session` fixture opens a connection, starts a transaction, then **overrides** `Dependencies.dynamic_engine` in the DI container to return that connection. All requests in the test reuse this connection. Each session built against it uses `join_transaction_mode="create_savepoint"`, so `auto_commit` releases the session's own savepoint while the outer transaction is rolled back in teardown, so DB state is clean between tests with no truncation needed. `app` and `client` fixtures build the real app and run it through `httpx.ASGITransport` + `asgi_lifespan.LifespanManager`. Polyfactory `SQLAlchemyFactory` is wired up via `set_async_session_in_base_sqlalchemy_factory`.
 
 **Migrations**: `migrations/env.py` reads `app.models.METADATA` and rewrites the DSN driver from `postgresql+asyncpg` → `postgresql` (Alembic uses sync psycopg2). Always run autogen against an upgraded DB — `just migration` enforces this.