Add SHELL_CREATED event with user and shell edges

[Copilot]

New SHELL_CREATED event kind with edges to the creating user and the created shell, following the existing event hook pattern.

Schema (tavern/internal/ent/schema/)

• event.go: Added SHELL_CREATED enum value, shell and user reverse edges
• shell.go: Added forward events edge
• user.go: Added forward events edge

Hook (tavern/internal/ent/hook_events.go)

• HookDeriveShellEvents(): On ShellMutation create, emits SHELL_CREATED event with shell + owner edges
• HookDeriveNotifications(): Broadcasts SHELL_CREATED notifications to all users (same pattern as HOST_ACCESS_NEW)

Registration

• tavern/config.go: client.Shell.Use(ent.HookDeriveShellEvents())

Frontend

• Updated enums.ts, interfacesQuery.ts, notification GQL query, and NotificationBell.tsx to render SHELL_CREATED events

Tests

• TestHookDeriveShellEvents: Verifies event creation with correct kind, shell/user edges, and notification generation

All generated code regenerated via go generate ./tavern/....