Fix PostMedia admin slow load caused by full Post queryset in change view

[Copilot]

The PostMediaAdmin change view was rendering the post FK as a standard <select>, forcing Django to load all Post records on every page open.

Change

• Added autocomplete_fields = ["post"] to PostMediaAdmin — replaces the full dropdown with an AJAX autocomplete widget that only queries on user input

# instagram/admin/post_media.py
class PostMediaAdmin(ModelAdmin):
    autocomplete_fields = ["post"]  # added

PostAdmin already defines search_fields, satisfying Django's autocomplete prerequisite.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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