Format traceback field to preserve indentation for readability

[p-r-a-v-i-n]

fixes #191

[p-r-a-v-i-n]

Render traceback using <pre> so whitespace and indentation are preserved.
Django admin collapses whitespace for TextField values by default.

[p-r-a-v-i-n]

Replace the raw traceback field with a formatted admin-only field to improve
readability without changing how the data is stored.

[p-r-a-v-i-n]

Used format_html() to safely render HTML while escaping the traceback content.

[p-r-a-v-i-n]

Before:

After:

[p-r-a-v-i-n]

not able to figure out why CI failed.

======================================================================
FAIL: test_interrupt_signals (tests.tests.test_database_backend.DatabaseWorkerProcessTestCase.test_interrupt_signals) [2]
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/runner/work/django-tasks/django-tasks/tests/tests/test_database_backend.py", line 1525, in test_interrupt_signals
    self.assertEqual(result.status, TaskResultStatus.RUNNING)
AssertionError: TaskResultStatus.READY != TaskResultStatus.RUNNING

----------------------------------------------------------------------
Ran 223 tests in 35.656s

FAILED (failures=1, skipped=8)
Preserving test database for alias 'default' ('/Users/runner/work/django-tasks/django-tasks/db-test.sqlite3')...
Used shuffle seed: 7410681148 (generated)
error: Recipe `test` failed on line 8 with exit code 1

[RealOrangeOne]

Question: Why would traceback not be in fields?

[p-r-a-v-i-n]

yeah, it will always be in fields.

[RealOrangeOne]

Issue: Inline styles aren't ideal - could this be moved to a small CSS file.

[p-r-a-v-i-n]

invoking Css file may be difficult. instead I will try it without css

[p-r-a-v-i-n]

IMO adding extra css does not make any difference here, so keeping simple. but happy iterate if require