Support SCM-specific user accounts for push identity resolution

[andypols]

Background
gitAccount was previously stored as a single value against the user.

It was not being used and was removed from the UI in #1507. However, #1604 later introduced a dependency on gitAccount when resolving push identities.

Because of the merge order, the current implementation from @coopernetes now relies on a value that can no longer be configured through the UI.

Problem
There is also a more fundamental issue with storing a single gitAccount against a user.

Some git-proxy installations have repositories across multiple SCM providers, and the same user may have a different username/account on each provider.

For example, a user could have one username in GitHub and another in GitLab.

A single user-level gitAccount therefore cannot reliably represent the user's SCM identity.

Proposed solution

• Replace the single gitAccount value with SCM-specific account data stored against the user.
• Update the UI to view and populate these SCM accounts.
• Expose identity resolution through a generic, extensible provider interface so additional SCM providers can support their own account mapping. Update the providers under src/service/gitProviders, introduced in feat: Refreshed UI #1507, so that provider-specific knowledge for resolving push identities lives within the relevant provider.
• Update checkUserPushPermission to use the new SCM-specific account structure.