Refactor `platform` table to enable custom domains

in order to support flexible platform support, the current `platform` table needs to not act like an enum (which it currently is) and instead act more like a form of data storage for specific user defined platforms (instances) of forges that are supported by CollectOSS.

Currently this table is referred to in these tables:
- message
- issue event
- pr event
- pr review

there may be a platform frontend api too but i cant find it

in all cases this just refers to the platform_id, which essentially is the same platform type flag that ContributorUUID uses.

This issue is for schema planning and migration to a better platform table that can adequately support the parent issue