Add multi-repo changelog support (link multiple GitHub repos per project)

Link multiple GitHub repositories to a single project changelog — Projects can now track one or more GitHub repositories. Users can add and remove repos from project settings, see per-repo sync status and last-synced times, and the background pipeline and webhooks now identify work by the specific project→repo link so a single repository can feed multiple projects.

Added a persistent project_repos join table to map one or more GitHub repositories to a single project changelog.database
Added a ProjectRepoRepository to manage repo links (create, list, update sync status, update branches, delete).backend
Added a Project Repositories settings UI so teams can add/remove repos, choose a tracked branch, and view per-repo sync status.frontend
Added API endpoints to list, add, and remove repo links for a project (GET/POST /v1/projects/:projectId/repos and DELETE /v1/projects/:projectId/repos/:repoId).backend

Validated installation ownership when linking a repo, emitted a repo/sync.requested event after adding a repo, and recorded repo.connected and repo.disconnected audit entries.backend
Prevented removing the last linked repo for a project (DELETE returns 422 if it would leave zero linked repos) and preserved all commits and changelog history when a repo link was removed.backend
Updated project creation to create a seed project_repos row in the same transaction and to include projectRepoId in the repo/sync.requested event.backend
Changed webhook handling to fan out one event per matching project_repo so a single push can trigger work for multiple projects that track the same repository.integrations

Carried projectRepoId through Inngest events (github/push.received, repo/sync.requested, commit/received) so pipeline stages identify the exact project→repo link that triggered work.backend
Moved repo-related methods off the ProjectRepository and into the new ProjectRepoRepository, and removed repo fields from the projects row and types.database
Updated web components (ProjectCard, ProjectGrid, ProjectSwitcher, project header and dashboard) to use ProjectRepoRow data and to gracefully handle projects with no linked repos.frontend
Added web proxy API routes and TanStack Query hooks (useProjectRepos, useAddProjectRepo, useRemoveProjectRepo) to support the new UI flows.frontend
Updated sync and push-processing workers to write sync status to project_repos and to use the trackedBranch stored on the project_repo row for branch filtering.backend
Adjusted github-sync-all-commits and resync flows to fan out sync requests per linked repo instead of per project.backend
Removed repo_owner/repo_name and repo-specific fields from project API responses and client-side project types.backend
Added a migration that back-filled a project_repos row for each existing project before dropping the old single-repo columns.database
Added comprehensive tests across database, API, and Inngest packages to cover the new repository linking behavior and worker changes.backend

Fixed a projectRepo variable scope bug in project creation so the repo create result is now assigned and projectRepoId can be included in events.backend

Documented the multi-repo changelog release and the new project_repos schema migration in the project changelog.