Fix AttributeError: django.utils.timezone has no attribute 'utc' in podcast details

[nakenyon]

Summary

• media_details_views.py line 426 used timezone.utc expecting Python's datetime.timezone, but timezone in that module's scope is django.utils.timezone (imported at the top of the file), which has no .utc attribute
• Fixed by importing timezone as dt_timezone from datetime alongside the existing datetime import inside the podcast details block, and using dt_timezone.utc for the epoch fallback value in the Coalesce annotation

Traceback

AttributeError: module 'django.utils.timezone' has no attribute 'utc'
  File "/yamtrack/app/media_details_views.py", line 426, in media_details
    Value(datetime(1970, 1, 1, tzinfo=timezone.utc),

Test plan

• Navigate to /details/pocketcasts/podcast/<id>/<slug> — should load without 500 error
• Confirm podcast episode list renders ordered by published date (newest first), with undated episodes at the end

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