test: disable TS background worker during migrations

[joseph-sentry]

we're getting the deadlock error in the API tests, the reason is that Timescale index creations deadlock with continuous aggregate refreshes like so:

2025-07-04 18:28:32.940 UTC [60026] DETAIL:  Process 60026 waits for AccessShareLock on relation 30135 of database 30126; blocked by process 60008.
        Process 60008 waits for ShareLock on relation 30150 of database 30126; blocked by process 60026.
        Process 60026: CALL _timescaledb_internal.policy_refresh_continuous_aggregate()
        Process 60008: 
                        CREATE INDEX IF NOT EXISTS ta_ts_testrun_summary_repo_id_ts_bin_idx
                        ON ta_timeseries_testrun_summary_1day (repo_id, timestamp_bin DESC);

so we have to disable the background worker while we run the migrations for tests

this PR takes overwrites the django_db_setup fixture and adds some statements to disable the background worker while we run migrations, similar to what we do in the sqlalchemy_db fixture in worker

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Codecov Report

Attention: Patch coverage is 85.71429% with 4 lines in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
apps/codecov-api/conftest.py	85.71%	4 Missing ⚠️

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[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 4 lines in your changes missing coverage. Please review.

Project coverage is 94.33%. Comparing base (8030fd0) to head (1c8c073).

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
apps/codecov-api/conftest.py	85.71%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #287      +/-   ##
==========================================
- Coverage   94.33%   94.33%   -0.01%     
==========================================
  Files        1222     1222              
  Lines       45309    45337      +28     
  Branches     1441     1441              
==========================================
+ Hits        42744    42768      +24     
- Misses       2264     2268       +4     
  Partials      301      301              

Flag	Coverage Δ
apiunit	96.47% <85.71%> (-0.02%)	⬇️

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[ElioDiNino]

Will be hard to confirm things are working for certain since I found this to be a flakey issue, but generally looks good. I would suggest deploying this to staging and rerunning the workflow a few times to make sure the deadlock isn't popping up.