feat(seer): Add new bulk+single Seer project repos endpoints

[srest2021]

fixes CW-1286

[linear-code]

CW-1286

[github-actions]

🚨 Warning: This pull request contains Frontend and Backend changes!

It's discouraged to make changes to Sentry's Frontend and Backend in a single pull request. The Frontend and Backend are not atomically deployed. If the changes are interdependent of each other, they must be separated into two pull requests and be made forward or backwards compatible, such that the Backend or Frontend can be safely deployed independently.

Have questions? Please ask in the #discuss-dev-infra channel.

[github-actions]

📊 Type Coverage Diff

✅ No new type safety issues introduced. Coverage: 93.56%

[sentry-warden]

IntegrityError on duplicate repository_id in replace_all_seer_project_repos

If repos_data contains duplicate repository_id values (not validated by the endpoint), get_or_create returns the same ProjectRepository on the second iteration and SeerProjectRepository.objects.create(project_repository=project_repo) raises an unhandled IntegrityError because SeerProjectRepository.project_repository has unique=True.

Evidence

• SeerProjectRepository.project_repository is declared unique=True in seer/models/project_repository.py.
• replace_all_seer_project_repos bulk-deletes existing records then calls SeerProjectRepository.objects.create(project_repository=project_repo, ...) inside a loop.
• ProjectRepository.objects.get_or_create(project=project, repository_id=data["repository_id"], ...) returns the same project_repo instance for each duplicate entry, so the second create violates the unique constraint.
• The PUT endpoint validates that all repository_id values belong to the org (via _get_valid_repo_ids) but does not check for duplicates within the submitted list.
• SeerProjectReposRequestSerializer / SeerProjectRepoSerializer also impose no uniqueness constraint on repository_id across the repos list.

Also found at 2 additional locations

• src/sentry/seer/endpoints/project_seer_repos.py:195-196
• src/sentry/seer/endpoints/project_seer_repos.py:270-283

_{Identified by Warden sentry-backend-bugs · SZM-EAL}