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/Validate.php

@@ -0,0 +1,46 @@
+<?php
+declare(strict_types=1);
+
+namespace AttHc\Server;
+
+final class Validate {
+    /** Hard exit 422 with a message. */
+    public static function fail(string $message): never {
+        Http::error(422, 'invalid', $message);
+        exit;
+    }
+
+    public static function requireString(array $body, string $key, int $maxLen = 1024): string {
+        if (!isset($body[$key]) || !is_string($body[$key]) || $body[$key] === '') {
+            self::fail("missing or empty string field: {$key}");
+        }
+        if (strlen($body[$key]) > $maxLen) self::fail("{$key} exceeds {$maxLen} chars");
+        return $body[$key];
+    }
+
+    public static function optionalString(array $body, string $key, int $maxLen = 1024): ?string {
+        if (!isset($body[$key])) return null;
+        if (!is_string($body[$key])) self::fail("{$key} must be a string");
+        if (strlen($body[$key]) > $maxLen) self::fail("{$key} exceeds {$maxLen} chars");
+        return $body[$key];
+    }
+
+    public static function requireInt(array $body, string $key): int {
+        if (!isset($body[$key]) || !is_int($body[$key])) {
+            self::fail("missing or non-int field: {$key}");
+        }
+        return $body[$key];
+    }
+
+    public static function optionalInt(array $body, string $key): ?int {
+        if (!isset($body[$key]) || $body[$key] === null) return null;
+        if (!is_int($body[$key])) self::fail("{$key} must be an integer");
+        return $body[$key];
+    }
+
+    public static function status(string $status): string {
+        $valid = ['not_started', 'done', 'skipped', 'blocked', 'n_a'];
+        if (!in_array($status, $valid, true)) self::fail('status must be one of ' . implode(', ', $valid));
+        return $status;
+    }
+}