feat: several Git repositories per project

A project keeps a list of repositories (api, web, admin…), each with its
own secret, instead of a single webhook secret. They all call the same
URL; the secret that verifies the signature tells which one it was, and
every commit, branch or pull request is linked with its repository's
name (shown on the ticket and in the MCP). Rotating or removing one
leaves the others alone, and what it linked stays.

A project that had turned the single webhook on keeps its secret, as a
repository named "principal".

API: the enable/disable endpoints and mutations give way to adding a
repository, rotating its secret and removing it (REST and GraphQL). The
web settings card lists the repositories. Docs, Word and PDF updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/web/tracker/infrastructure/src/lib/graphql/fragments.graphql

@@ -120,6 +120,7 @@ fragment IssueFields on Issue {
     ref
     state
     provider
+    repository
     author
     at
   }