Move has_duplicate functionality to Media base class

[johannaengland]

Scope and purpose

Very similar to #2015. Basically copied from #936.

Contributor Checklist

Every pull request should have this checklist filled out, no matter how small it is.
More information about contributing to Argus can be found in the
Development docs.

• Added a changelog fragment for towncrier
• Added/amended tests for new/changed code
• Added/changed documentation, including updates to the user manual if feature flow or UI is considerably changed
• Linted/formatted the code with ruff and djLint, easiest by using pre-commit
• The first line of the commit message continues the sentence "If applied, this commit will ...", starts with a capital letter, does not end with punctuation and is 50 characters or less long. See our how-to
• If applicable: Created new issues if this PR does not fix the issue completely/there is further work to be done
• If this results in changes in the UI: Added screenshots of the before and after
• If this results in changes to the database model: Updated the ER diagram

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.02%. Comparing base (7ef8415) to head (72b7761).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2019      +/-   ##
==========================================
+ Coverage   88.97%   89.02%   +0.05%     
==========================================
  Files         155      155              
  Lines        7339     7329      -10     
==========================================
- Hits         6530     6525       -5     
+ Misses        809      804       -5     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[johannaengland]

I noticed when testing this that if I try to add a duplicate destination I get the following error:

2026-08-03 14:40:21,535 django.request ERROR    Internal Server Error: /notifications/destinations/create/
Traceback (most recent call last):
  File "/home/johanna/Argus/.venv/lib/python3.12/site-packages/django/core/handlers/exception.py", line 55, in inner
    response = get_response(request)
               ^^^^^^^^^^^^^^^^^^^^^
  File "/home/johanna/Argus/.venv/lib/python3.12/site-packages/django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/johanna/Argus/.venv/lib/python3.12/site-packages/django/views/generic/base.py", line 105, in view
    return self.dispatch(request, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/johanna/Argus/.venv/lib/python3.12/site-packages/django/views/generic/base.py", line 144, in dispatch
    return handler(request, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/johanna/Argus/.venv/lib/python3.12/site-packages/django/views/generic/edit.py", line 182, in post
    return super().post(request, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/johanna/Argus/.venv/lib/python3.12/site-packages/django/views/generic/edit.py", line 151, in post
    return self.form_valid(form)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/home/johanna/Argus/src/argus/htmx/destination/views.py", line 101, in form_valid
    form.save()
  File "/home/johanna/Argus/src/argus/htmx/destination/forms.py", line 39, in save
    self.serializer.save(user=self.request.user)
  File "/home/johanna/Argus/.venv/lib/python3.12/site-packages/rest_framework/serializers.py", line 182, in save
    assert not self.errors, (
           ^^^^^^^^^^^^^^^
AssertionError: You cannot call `.save()` on a serializer with invalid data.
2026-08-03 14:40:21,538 django.server INFO     "POST /notifications/destinations/create/ HTTP/1.1" 200 655

I will work on fixing that in a follow up issue, since this error already exists on main

[github-actions]

Test results

    8 files  1 792 suites   3m 17s ⏱️
1 005 tests 1 004 ✅ 1 💤 0 ❌
8 040 runs  8 032 ✅ 8 💤 0 ❌

Results for commit 72b7761.