test: remove legacy Flask unittest suite (tests/backend)

[prabinoid]

What type of PR is this? (check all applicable)

• 🍕 Feature
• 🐛 Bug Fix
• 📝 Documentation
• 🧑‍💻 Refactor
• ✅ Test
• ❓ Other (please specify)

Related Issue

Related to #7183

Describe this PR

Removes the legacy Flask-based unittest suite under tests/backend/
(139 files). It was superseded by the async pytest suite in tests/api/,
which is what CI actually runs (pr_test_backend.yml).

Two tests/api modules still imported helpers from the old tree, so a naive
delete would have broken CI. Handled first:

• Repointed get_canned_osm_user_json_details and get_canned_json imports
  from tests.backend.helpers → tests.api.helpers (identical functions; the
  required fixtures already exist in tests/api/helpers/test_files/).
• Updated the Makefile test-backend target to pytest tests/api
  (and corrected the stale backend compose service name to tm-backend).
• Updated docs/developers/development-setup.md and submit-pr.md to the
  pytest tests/api commands.
• Deleted tests/backend/.

[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

[jayaddison]

@prabinoid NB: the TestProjectRecommendationService integration test cases from the tests/backend test suite don't seem to be present in the tests/api suite -- a test_recommendation_service.py file was created, but is empty.

Maybe that shouldn't block work the dependency refresh work, but I thought it'd be worth mentioning in case those tests should be ported across too.

[prabinoid]

Yes, thank you @jayaddison it must have missed during the migration. I will check and update it. Thanks!

[jayaddison]

You're welcome - thank you!