Add use_session_state hook for persistent state across WebSocket reconnects - #300

Description

Add a dedicated, database-backed hook reactpy_django.hooks.use_session_state(default, key) that persists state so it survives WebSocket reconnects (and, optionally, page reloads).

Problem

On a WebSocket reconnect the client re-sends mount-component, the server creates a fresh Layout, and all in-memory component state (use_state, etc.) is lost. This is especially problematic for deployments that use multiple processes or reactpy-django's built-in round-robin load balancing across hosts, where a reconnect may land on a different worker whose memory does not contain the previous state.

Solution

Add a database-backed hook so state is durable across process restarts and round-robin hosts, unlike an in-memory approach.

from reactpy_django import hooks

def my_component():
    count, set_count = hooks.use_session_state(0, key="counter")
    return html.button({"onClick": lambda _: set_count(count + 1)}, f"Count: {count}")

Design decisions

• DB-backed (not in-memory) so state is durable across multi-process workers and round-robin hosts.
• Stable scope identity controlled by REACTPY_SESSION_STATE_MODE:
  • "tab" (default): scoped to the rendered component's UUID (a per-tab/per-component token that is stable across reconnects). Works for anonymous users without django.contrib.sessions and isolates state between browser tabs.
  • "user": scoped to the authenticated user's PK, falling back to a per-tab token for anonymous users.
• Debounced, periodic writes (REACTPY_SESSION_STATE_SYNC_INTERVAL, default 10s) so rapid state changes (e.g. typing) are coalesced into one DB write; state is also flushed on unmount so the latest value is persisted before a reconnect.
• Serialized with dill (already a dependency) to support arbitrary serializable Python objects.

Settings added

Notes for reviewers

• The per-tab identity is currently derived from the component UUID generated by the {% reactpy %} template tag. This is stable across reconnects (baked into the rendered HTML) but not across full page reloads. A future enhancement could lift the per-tab token into ReactPy core for a more stable, browser-persistent identity.
• Tests are provided for the database layer. Hook integration tests (via Playwright) and full documentation are follow-ups; this PR focuses on the implementation and CI correctness.

Checklist

Please update this checklist as you complete each item:

• Tests have been developed for bug fixes or new functionality.
• The changelog has been updated, if necessary.
• Documentation has been updated, if necessary.
• GitHub Issues closed by this PR have been linked.

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