Integrate Django flash messages into Inertia page props

[benaduo]

Summary

• Integrated Django's built-in flash messages framework into the
  Inertia response cycle so messages are automatically available
  as page props on the frontend
• Messages are added after partial render filtering to prevent
  permanent message loss on partial page requests
• Messages key is omitted entirely when no messages exist, keeping
  props clean

Motivation

Fixes Issue #38

In a standard Django app, flash messages set via messages.success(),
messages.error() etc. are consumed by the template layer. Since
Inertia bypasses Django templates and communicates entirely through
JSON props, these messages were silently lost — they were never
included in the Inertia response, so the frontend had no way to
display them.

Changes

• inertia/http.py: Import get_messages and add flash messages to
  props inside build_props(), after the partial render filtering step
• inertia/tests/testapp/views.py: Add messages_test and
  no_messages_test views for testing
• inertia/tests/testapp/urls.py: Register the two new test URLs
• inertia/tests/test_messages.py: 3 tests covering message inclusion,
  empty props behavior, and message consumption after render

Frontend Usage

Once merged, flash messages are available in props automatically:

// React / Vue / Svelte
const { messages } = usePage().props

// Each message has:
// { level: "success" | "error" | "warning" | "info", message: "..." }