Central history server + plugin write-through sync (epic hc-0p1)

Adds a PHP/SQLite history server in server/ and refactors the plugin to
write every session change through it. Healthcheck history now survives
plugin uninstall and groups across dev + live URLs for the same engagement
via an editable site_key (defaults to the normalised host).

Server (server/):
- Front controller + hand-rolled autoloader, no framework, no composer
- SQLite default DSN; swap to MySQL by changing config.php
- Schema: healthchecks (PK id, UNIQUE (site_key, started_at)) + step_updates
  (PK (healthcheck_id, step_id)) + request_log; auto-migration runner
- 8 endpoints: POST/GET/PUT healthchecks, PUT/GET step rows, GET step history
  with exclude_id, GET /sites (recent), GET /step-counts (badge data)
- Bearer auth via hash_equals; HTTPS expected (plugin enforces client-side)
- DEPLOY.md with Apache/nginx vhosts, Let's Encrypt, SQLite backup cron,
  and the /home/www/ perm gotcha
- dev-router.php works around PHP -S 405-ing dotted uniqid paths

Plugin:
- ATT_HC_Api HTTP client reads ATT_HC_API_URL/ATT_HC_API_KEY constants
  from wp-config.php; refuses non-HTTPS with a loopback dev exception
- ATT_HC_Session is now write-through: every start/update_step/finish/
  set_autocheck POSTs or PUTs to the server first, then updates the local
  WP option cache. No drift possible — failures throw ATT_HC_Api_Exception
- previous() now reads from /healthchecks?include=steps and reconstructs;
  the old att_hc_previous_session local option is gone
- ATT_HC_Session::resume(id) hydrates a server session into the local cache
- Start screen: editable site_key (defaults to normalise_site_url()),
  datalist of recent engagements, table of in-progress sessions for the
  chosen key with Resume buttons. Double-click guard on start + resume
  handlers short-circuits if a session is already active
- Per-step <details> disclosure shows "Previous notes (N)" badge from
  /step-counts; lazy-loads detail rows on first expand via admin-ajax,
  caches via data-loaded, resets on error so user can retry
- All admin handlers catch ATT_HC_Api_Exception and surface via
  att_hc_api_error transient → admin notice
- Hard config-error gate at the top of the admin page blocks the UI when
  ATT_HC_API_URL/ATT_HC_API_KEY are missing or malformed

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/src/bootstrap.php

@@ -0,0 +1,40 @@
+<?php
+declare(strict_types=1);
+
+namespace AttHc\Server;
+
+// Hand-rolled PSR-4-ish autoloader. No composer dependency.
+spl_autoload_register(static function (string $class): void {
+    $prefix = 'AttHc\\Server\\';
+    if (strncmp($class, $prefix, strlen($prefix)) !== 0) return;
+    $relative = substr($class, strlen($prefix));
+    $path = __DIR__ . '/' . str_replace('\\', '/', $relative) . '.php';
+    if (is_file($path)) require $path;
+});
+
+error_reporting(E_ALL);
+ini_set('display_errors', '0');
+ini_set('log_errors', '1');
+
+set_error_handler(static function (int $severity, string $message, string $file, int $line): bool {
+    if (!(error_reporting() & $severity)) return false;
+    throw new \ErrorException($message, 0, $severity, $file, $line);
+});
+
+set_exception_handler(static function (\Throwable $e): void {
+    error_log('[att-hc-server] ' . $e::class . ': ' . $e->getMessage() . ' @ ' . $e->getFile() . ':' . $e->getLine());
+    if (!headers_sent()) {
+        http_response_code(500);
+        header('Content-Type: application/json');
+    }
+    echo json_encode(['error' => 'internal server error', 'code' => 'internal']);
+});
+
+$configPath = __DIR__ . '/../config.php';
+if (!is_file($configPath)) {
+    http_response_code(500);
+    header('Content-Type: application/json');
+    echo json_encode(['error' => 'server not configured: missing config.php', 'code' => 'no_config']);
+    exit;
+}
+Config::load(require $configPath);