[APIView] Convert PR Revision to Automatic instead of maintaining one of each

Currently when a PR API Revision is merged into main, the PR Revision is retained AND an Automatic (from Main/source of truth) revision is created. To avoid this duplication, we should convert the PR Revision to an Automatic Revision via flag in the database. Once the PR Revision is converted to Automatic it will be locked and cannot be deleted.

Start by tracing how PR API Revisions are merged into main and where the revision type is stored in the database. Verify the conversion preserves the revision while marking it Automatic and prevents deletion afterward; no files or tests are named in the issue.

Written by the indexing model from the issue text.