feat: rework async event system (#196)

* feat: rework async event system

* docs: add v3 migration guide

* feat: add cache

* enhancements

docs/migration/v3.md

@@ -0,0 +1,93 @@
+# Migrating to v3.0
+
+This page covers every breaking change introduced in **v3.0** and the steps required to update your code.
+
+---
+
+## Models
+
+The lifecycle event system has been rewritten. Callbacks are now registered with a module-level [`listens_for`](../reference/models.md#fastapi_toolsets.models.listens_for) decorator and dispatched by [`EventSession`](../reference/models.md#fastapi_toolsets.models.EventSession), replacing the mixin-based approach from `v2`.
+
+### `WatchedFieldsMixin` and `@watch` removed
+
+Importing `WatchedFieldsMixin` or `watch` will raise `ImportError`.
+
+Model method callbacks (`on_create`, `on_delete`, `on_update`) and the `@watch` decorator are replaced by:
+
+1. **`__watched_fields__`** — a plain class attribute to restrict which field changes trigger `UPDATE` events (replaces `@watch`).
+2. **`@listens_for`** — a module-level decorator to register callbacks for one or more [`ModelEvent`](../reference/models.md#fastapi_toolsets.models.ModelEvent) types (replaces `on_create` / `on_delete` / `on_update` methods).
+
+=== "Before (`v2`)"
+
+    ```python
+    from fastapi_toolsets.models import WatchedFieldsMixin, watch
+
+    @watch("status")
+    class Order(Base, UUIDMixin, WatchedFieldsMixin):
+        __tablename__ = "orders"
+
+        status: Mapped[str]
+
+        async def on_create(self):
+            await notify_new_order(self.id)
+
+        async def on_update(self, changes):
+            if "status" in changes:
+                await notify_status_change(self.id, changes["status"])
+
+        async def on_delete(self):
+            await notify_order_cancelled(self.id)
+    ```
+
+=== "Now (`v3`)"
+
+    ```python
+    from fastapi_toolsets.models import ModelEvent, UUIDMixin, listens_for
+
+    class Order(Base, UUIDMixin):
+        __tablename__ = "orders"
+        __watched_fields__ = ("status",)
+
+        status: Mapped[str]
+
+    @listens_for(Order, [ModelEvent.CREATE])
+    async def on_order_created(order: Order, event_type: ModelEvent, changes: None):
+        await notify_new_order(order.id)
+
+    @listens_for(Order, [ModelEvent.UPDATE])
+    async def on_order_updated(order: Order, event_type: ModelEvent, changes: dict):
+        if "status" in changes:
+            await notify_status_change(order.id, changes["status"])
+
+    @listens_for(Order, [ModelEvent.DELETE])
+    async def on_order_deleted(order: Order, event_type: ModelEvent, changes: None):
+        await notify_order_cancelled(order.id)
+    ```
+
+### `EventSession` now required
+
+Without `EventSession`, lifecycle callbacks will silently stop firing.
+
+Callbacks are now dispatched inside `EventSession.commit()` rather than via background tasks. Pass it as the session class when creating your session factory:
+
+=== "Before (`v2`)"
+
+    ```python
+    from sqlalchemy.ext.asyncio import async_sessionmaker, create_async_engine
+
+    engine = create_async_engine("postgresql+asyncpg://...")
+    SessionLocal = async_sessionmaker(engine, expire_on_commit=False)
+    ```
+
+=== "Now (`v3`)"
+
+    ```python
+    from sqlalchemy.ext.asyncio import async_sessionmaker, create_async_engine
+    from fastapi_toolsets.models import EventSession
+
+    engine = create_async_engine("postgresql+asyncpg://...")
+    SessionLocal = async_sessionmaker(engine, expire_on_commit=False, class_=EventSession)
+    ```
+
+!!! note
+    If you use `create_db_session` from `fastapi_toolsets.pytest`, the session already uses `EventSession` — no changes needed in tests.