Add CustomApp types, the gc_custom_apps Drizzle schema, and its migration

[rudokemper]

Goal

Closes #96.

Screenshots

n/a

What I changed and why

• Added gc_custom_apps Drizzle schema + migration.
• Added shared CustomApp types/validation helpers and server list/replace utilities for the upcoming admin API and homepage grid.

How I convinced myself this is right

Plan validation, and following existing Drizzle / migration conventions.

What I'm not doing here

LLM use disclosure

Cursor Grok 4.5 implementation based on plan. I didn't spot anything that needed fixing.

[conservationtimothy]

We are importing just to export, not sure how I feel but maybe it's inevitable.

[conservationtimothy]

Could simplify this slightly: Drizzle transactions can return a value, and Postgres supports .returning(), so we can return the transaction directly and map the inserted rows instead of doing a second listCustomApps() query afterward. See [Drizzle transactions](https://orm.drizzle.team/docs/transactions) and [Postgres returning](https://orm.drizzle.team/docs/insert#insert-returning).

So something like:

const rows = await tx
    .insert(schema.gcCustomApps)
    .values(
      apps.map((app, index) => ({
        id: app.id,
        name: app.name,
        description: app.description,
        iconUrl: app.iconUrl,
        tags: app.tags,
        subdomain: app.subdomain,
        enabled: app.enabled,
        sortOrder: index,
        createdAt: now,
        updatedAt: now,
      })),
    )
    .returning();

  return rows.map(mapCustomAppRow);

[rudokemper]

Thanks, agreed this is an improvement. Implemented in 3808078