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>

includes/class-att-hc-api-exception.php

@@ -0,0 +1,17 @@
+<?php
+if (!defined('ABSPATH')) exit;
+
+/**
+ * Thrown by ATT_HC_Api when the central server returns a non-2xx or is unreachable.
+ * Callers should catch and surface the message via admin notices — never silently swallow.
+ */
+final class ATT_HC_Api_Exception extends Exception {
+    public string $error_code;
+    public int $http_status;
+
+    public function __construct(string $message, string $error_code = 'unknown', int $http_status = 0) {
+        parent::__construct($message);
+        $this->error_code  = $error_code;
+        $this->http_status = $http_status;
+    }
+}