Refactor: Supabase admin migration (#273)

<!-- CURSOR_SUMMARY -->
> [!NOTE]
> **High Risk**
> High risk because it rewires team resolution/authorization to be
slug-based across dashboard routes and API handlers, including
auth/session plumbing and new repository layers; mistakes could break
access control or routing for existing users.
> 
> **Overview**
> Moves the app to **slug-only team routing**: dashboard pages, redirect
resolvers (e.g. `/dashboard`, `/dashboard/account`), and team-scoped API
routes now take `teamSlug` and resolve/authorize to a team ID via
`getTeamIdFromSlug`/`resolveUserTeam`, removing ID fallbacks and
hardening tab redirects with `Object.hasOwn`.
> 
> Introduces a set of `core/modules/*` repositories and models (auth,
teams, sandboxes, builds, keys, billing, support) and updates
imports/tests to use the new `core/shared` clients/contracts paths; adds
a client-side `DashboardTeamGate` with prefetched `teams.list` to
enforce access in the new `dashboard/[teamSlug]/layout`.
> 
> Removes several old SQL migration files and shifts the `/api/health`
check from a direct Supabase query to calling the dashboard API
`/health` endpoint.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
ab45405. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: ben-fornefeld