feat(github): per-repo "Bind to project" button on settings page

[ralyodio]

Summary

The repo list at `/settings/integrations/github` was read-only — users could see all 200+ repos but the only clickable thing was an external link to github.com. After a fresh install, that page was a dead end.

Now each row has a Bind to project ▾ dropdown. Pick a project → row updates to Bound to (or "Bound to N projects" if you bind to several). Same data as the per-project Repos tab, just exposed from the GitHub-side surface so users who arrive via the install flow can take the next step without hunting.

Pieces

• `bind-button.tsx` — client component. Dropdown of the user's projects with ✓ marks for already-bound ones. Toggling fires POST or DELETE on the existing `/api/projects/[id]/repos` endpoints. Optimistic local state + `router.refresh()` after each change.
• `repos-filter.tsx` — now accepts `projects` and `bindings` and renders `BindButton` per row. Slices bindings to the right repo via an `owner/name` map (O(1) lookup).
• Page pre-loads the user's projects and all of their `project_repos` rows once (RLS-scoped), then partitions per installation when rendering.

Test plan

After deploy:

• Visit `/settings/integrations/github` → confirm each repo row has a button on the right ("Bind to project ▾" when none, "Bound to " when one).
• Click the button → dropdown shows all your projects. Each row has ✓ bound or + bind.
• Bind: status flips to ✓ bound, button label updates to "Bound to ".
• Visit the project's Repos tab → confirm the repo appears in the bound list.
• Unbind from the dropdown → repo disappears from the project's bound list.
• On a user with zero projects: button shows "Create a project first" instead of the dropdown.

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