Add multi-server-per-admin support via sa_admins_servers table

[Ravid-A]

Summary

Admin records are bound to a single server through the sa_admins.server_id column (NULL = global). Granting one admin to several specific servers means duplicating the whole admin row (plus its flag rows) per server, and "all servers" is modelled as a NULL foreign-key value, which is awkward to query and easy to get wrong.

This PR introduces a sa_admins_servers link table — mirroring the existing sa_groups / sa_groups_servers pattern — so a single admin record can be attached to many servers. It also adds an explicit global flag on sa_admins for all-servers admins, replacing the NULL-server_id convention. Existing data is migrated automatically and behaviour is preserved.

Changes

All changes are additive/mechanical against eea700b.

• Migration 017_CreateAdminsServersTable.sql (MySQL + SQLite) — creates sa_admins_servers (id, admin_id, server_id NOT NULL, FK admin_id → sa_admins(id) ON DELETE CASCADE); adds global column to sa_admins (default 0); backfills (server-specific admins → one link row each, old NULL-server admins → global = 1); then drops sa_admins.server_id. Both new files are registered in the .csproj for CopyToOutputDirectory, matching every other migration.
• IDatabaseProvider — adds GetAddAdminServerQuery() and GetDeleteOrphanedAdminsQuery().
• MysqlDatabaseProvider / SqliteDatabaseProvider (symmetric):
  • GetAdminsQuery — now selects admins where global = 1 OR a sa_admins_servers row exists for the current server (via EXISTS, so the flag join doesn't multiply rows). Previously filtered on sa_admins.server_id.
  • GetAddAdminQuery — inserts the global column instead of server_id.
  • GetAddAdminServerQuery — inserts the per-server link row.
  • GetDeleteAdminQuery(globalDelete: false) — deletes only the current server's link row, leaving the admin and its other server assignments intact. Global delete still removes the sa_admins row (cascading the links).
  • GetDeleteOrphanedAdminsQuery — deletes non-global admins that have no remaining sa_admins_servers rows (see Notes).
• PermissionManager — AddAdminBySteamId: -g sets global = 1 and writes no link row (applies everywhere); otherwise global = 0 and a link row is written for the current server. New DeleteOrphanedAdmins() runs the orphan-cleanup query.
• PlayerManager — DeleteOrphanedAdmins() is added to the existing expire-timer task batch (alongside DeleteOldAdmins()), so orphans are swept on the same cadence as expired bans/mutes/warns.
• The global identifier is a MySQL reserved word, so it is backticked in every statement.

Test plan

• SQLite migration backfill — synthetic sa_admins (one NULL-server, two server-specific) run through 017: NULL admin → global = 1 with no link row; server-specific admins → one link row each with correct server_id; sa_admins.server_id dropped; sa_admins_servers.server_id enforced NOT NULL. (SQLite 3.50.4 via System.Data.SQLite.Core 1.0.119.)
• SQLite load query — GetAdminsQuery returns, per server: server 5 → global + server-5 admin; server 7 → global + server-7 admin; unregistered server 9 → global only.
• SQLite orphan sweep — after a non-global delete strips an admin's last link, GetDeleteOrphanedAdminsQuery removes exactly that admin (global = 0, no links) while leaving the global admin and a still-linked admin untouched.
• dotnet build (Rebuild, net8.0) — clean, 0 warnings / 0 errors.
• MySQL — not retested by submitter (no MySQL test env handy). The SQL mirrors the existing sa_groups_servers patterns and existing admin queries; EXISTS, the global backfill, DROP COLUMN, and the orphan DELETE … NOT IN (subquery on a different table) are all standard MySQL. A second pair of eyes on a MySQL deployment before merge would be welcome.
• Live in-game smoke test — recommend a quick css_addadmin (with and without -g) and css_deladmin on a running server at merge time to confirm permission load/unload end-to-end.

Notes

• Backward compatible. Existing global and server-specific admins keep working unchanged after the migration; no manual data steps required.
• Orphan handling. A non-global delete only removes the current server's link row, so an admin can be left with global = 0 and zero links — invisible to every server (never loaded) but still occupying rows. DeleteOrphanedAdmins() sweeps these on the expire timer so they don't accumulate.
• SQLite FK cascade caveat (pre-existing, unchanged). SQLite connections in SqliteDatabaseProvider don't issue PRAGMA foreign_keys = ON, so ON DELETE CASCADE doesn't fire on SQLite — exactly as with the existing sa_admins_flags table. This means an orphaned admin's sa_admins_flags rows can linger on SQLite after the admin row is swept; MySQL cascades them. Enabling the pragma globally would fix this class of issue but is a separate cleanup, out of scope here.
• The globalAdmin/-g plumbing in basecommands.cs is left as-is; it now drives the global column instead of a NULL server_id.

Tested SA version: eea700b. Submitted from Ravid-A:feature/multi-server-per-admin.