GroupVerify 0.2.0: Steam group / override verification with an API for other plugins

Players are verified when they're in the configured Steam group (public memberslistxml,
refetched every 10 minutes, kept on failure) or have an unexpired override row in
gv_verified, written by the web panel. Settings and data live in CS2-SimpleAdmin's MySQL
database (gv_verified, gv_settings, gv_migrations), reached through its API.

- css_gv_group <groupId64|0>, css_gv_enable, css_gv_disable (@css/root): verification runs
  only when enabled with a group set.
- css_verify <target>: why a player is or isn't verified. Admin and above (by immunity from
  SimpleAdmin's groups) plus Guardian, unless admin_overrides.json says otherwise.
- groupverify:api (shared/GroupVerifyApi): IsVerified, GetStatus, IsActive and
  OnVerifiedChanged. The API object lives in the shared assembly, so it survives hot
  reloads of GroupVerify.

Migrations/001_CreateTables.sql

@@ -0,0 +1,29 @@
+-- GroupVerify's tables, in CS2-SimpleAdmin's database. Applied once at load, tracked in
+-- gv_migrations (like SimpleAdmin's sa_migrations, kept apart so neither confuses the other).
+-- Times are wall-clock values in gv_settings' timezone, which should match SimpleAdmin's Timezone.
+
+-- One row per player and source. A player counts as verified if they're in the Steam group's
+-- member list, or have a row with `override` = 1 whose expires_at is NULL (never) or still ahead.
+--   source 'group':  written by the plugin for everyone in the group list, `override` 0. Removed
+--                    when they leave the group. The live list decides, not these rows.
+--   source 'manual': written by the web panel, `override` 1, expires_at NULL or a date.
+-- updated_at is when the plugin last saw a group row in the list.
+CREATE TABLE IF NOT EXISTS `gv_verified` (
+    `steamid64` bigint(20) NOT NULL,
+    `source` enum('group','manual') NOT NULL,
+    `override` tinyint(1) NOT NULL DEFAULT 0,
+    `expires_at` datetime NULL DEFAULT NULL,
+    `added_by` varchar(128) NOT NULL DEFAULT '',
+    `created_at` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    `updated_at` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    PRIMARY KEY (`steamid64`, `source`),
+    KEY `idx_gv_verified_override` (`override`, `expires_at`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;
+
+-- Plugin settings, name -> value. The plugin adds any missing name with its default at load, and
+-- rereads them on every database reload, so edits apply without a restart.
+CREATE TABLE IF NOT EXISTS `gv_settings` (
+    `name` varchar(64) NOT NULL,
+    `value` varchar(255) NOT NULL DEFAULT '',
+    PRIMARY KEY (`name`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;