Add MCP identity mapping and read tools (#111, phase 4)

MCP requests now run as the signed-in Tinker Tickets user, under exactly
the same rules as the web login, and expose the first two tools:
search_tickets and get_ticket.

Identity:
- IdentityMiddleware maps the validated token to a user with the same
  checks as AuthMiddleware: the admin/employee group rule, now extracted
  into helpers/AccessPolicy.php so both entry points share one copy, then
  UserModel::syncUserFromAuthelia(), which creates/updates the row and
  derives is_admin from groups.
- Claims are read from the validated token's server-side PSR-7 request
  attributes, not from JSON-RPC _meta. The SDK's OAuthRequestMetaMiddleware
  is deliberately not used: it array_merges into client-writable _meta,
  so only the keys the validator happens to set are overwritten and a
  client could inject others.

Scopes (ToolScopeMiddleware), enforced before dispatch:
- lifecycle messages need only a valid token; write tools (listed in
  ToolCatalog, the single registry) need tickets:write; everything else
  needs tickets:read, which tickets:write implies.
- Denials are the spec's step-up challenge: 403 +
  WWW-Authenticate: Bearer error="insufficient_scope", scope=...,
  resource_metadata=...

Tools (read-only, annotated readOnlyHint):
- search_tickets: text/status/priority/category/assignee ("me",
  "unassigned", or a username), paginated, via TicketModel::getAllTickets
  with the user's visibility filter. Defaults to every non-Closed status.
- get_ticket: details + comments, gated by canUserAccessTicket. A missing
  ticket and a non-visible one return the same "not found".

Verified locally against a real MariaDB fixture (public, confidential,
internal+group, and closed tickets across two users), driving the real
pipeline (ToolCatalog, both middlewares, SDK transport) with only JWT
validation stubbed: 20/20 checks pass, including visibility parity per
user, confidential tickets hidden from non-owners, the group check
rejecting a user without admin/employee, a missing preferred_username
rejected, 403 insufficient_scope for a token without tickets:*, and
write implying read. Also exercised the stateless 2026-07-28 era (no
session, _meta + MCP-Protocol-Version/Mcp-Method/Mcp-Name headers), which
returns the same visibility-filtered results.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MGDKHiU5RJdo3dqQUDow3X

middleware/AuthMiddleware.php

@@ -263,21 +263,8 @@ class AuthMiddleware
      */
     private function checkGroupAccess($groups)
     {
-        if (empty($groups)) {
-            return false;
-        }
-
-        // Check for admin or employee group membership
-        // Filter to safe characters only to prevent header injection attacks
-        $userGroups = array_filter(
-            array_map('trim', explode(',', strtolower($groups))),
-            function ($g) {
-                return preg_match('/^[a-z0-9_\-]+$/', $g);
-            }
-        );
-        $requiredGroups = ['admin', 'employee'];
-
-        return !empty(array_intersect($userGroups, $requiredGroups));
+        require_once dirname(__DIR__) . '/helpers/AccessPolicy.php';
+        return AccessPolicy::hasAppAccess((string)($groups ?? ''));
     }
 
     /**