feat: add endpoint to list disabled users with cursor-based pagination

[francismars]

Summary

This PR adds an admin endpoint in pubky-homeserver to list disabled users with cursor pagination.

• Added GET /users/disabled to the admin protected router.
• Added handler/query/response support for disabled users listing.
• Added repository-level disabled-users pagination with deterministic ordering.

API contract

Endpoint

GET /users/disabled

Auth

Uses existing admin middleware (X-Admin-Password header).

Query params

• limit?: u16
• cursor?: string (z32 pubkey, non-inclusive)

Response

{
  "items": [{ "pubkey": "..." }],
  "next_cursor": "..." | null
}

Error behavior

• Invalid cursor => 400 Bad Request (Invalid cursor)

Implementation details

• Route wiring:
  • pubky-homeserver/src/admin_server/app.rs
• Admin handler + response/query types:
  • pubky-homeserver/src/admin_server/routes/disable_users.rs
• Persistence query + pagination:
  • pubky-homeserver/src/persistence/sql/entities/user.rs

Pagination behavior:

• Filter on disabled = true
• Deterministic order by public_key ascending
• Cursor is non-inclusive (public_key > cursor)
• Uses limit + 1 fetch to determine next_cursor

Test plan

• cargo fmt
• cargo check -p pubky-homeserver
• Manual Umbrel validation:
  • Seeded users in Postgres (2 disabled, 1 enabled)
  • GET /users/disabled?limit=1 returned one disabled user + non-null next_cursor
  • Request with returned cursor returned remaining disabled user(s) + next_cursor: null
  • Invalid cursor returned 400
  • Enabled user was not returned