fix(webhook): match manual webhook repositories exactly

The manual webhook handlers selected target applications with a
`git_repository LIKE %full_name%` substring query, so a payload
repository name could match unintended applications when repository
names overlap.

Add a `MatchesManualWebhookApplications` trait that validates the
incoming `owner/repo` value and matches `Application.git_repository`
by exact normalized path. Github, Gitlab, Gitea and Bitbucket manual
handlers now use it, reject invalid repository input early, and return
a consistent generic webhook failure payload.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>