This change adds a new module for converting legacy per-repository bug tracker state into the new configuration model. It classifies each repository (in-repo, standalone tracker service, or custom URL), extracts unprefixed settings, and creates `ConfiguredBugTracker` configurations scoped to the repositories that produced them. Identical standalone and custom configurations are deduplicated by fingerprint. These fingerprints never span Local Sites. Each created configuration records its fingerprint in extra_data, making the creation idempotent. In-repo configurations aren't deduplicated, because they're bound to individual repositories anyway. A repository's default bug tracker is only assigned when unset or when it was itself derived from legacy settings, so manual edits are never overwritten. The legacy fields are left untouched. Nothing calls this yet. The save-time sync, management command, and upgrade step follow in later changes. Testing Done: Ran unit tests.