Verify who connects GitHub, GitLab or Bitbucket

Connecting a Git provider now confirms that the person connecting it really controls that account, so an organization can only link repositories its own admins own.

• Improved GitHub connections: TofuPilot checks on GitHub that you own the installation, either as the account itself or as an owner of the GitHub organization, and that the connection was started from your own browser.
• Improved GitLab and Bitbucket connections: each group or project is claimed per server, so organizations on different self-managed servers no longer block each other, and GitLab now requires a Maintainer token as documented.
• Improved how account emails are sent, so only TofuPilot itself can trigger them.
• Updated the self-hosted GitHub setup: add the App's client ID and client secret and enable user authorization during installation before connecting GitHub; GitLab tokens below Maintainer need replacing when they next expire.