Fix session_start guards, add missing API routes, rewrite README

- Added session_status() === PHP_SESSION_NONE guard to six API files
  (custom_fields, revoke_api_key, manage_templates, generate_api_key,
  get_template, manage_recurring) that called bare session_start() after
  RateLimitMiddleware had already started the session
- Registered /api/notifications.php and /api/user_avatar.php in index.php
  router (were missing, served only by direct file access)
- Complete README rewrite: remove all Discord references (Matrix/hookshot
  is the only external notification method), add hwmonDaemon API docs,
  document all TDS v1.2 features (kanban, charts, SLA, command palette,
  notification bell, watcher avatars, @mention, etc.), fix keyboard
  shortcuts table, add Matrix/LDAP env vars to setup section

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/get_template.php

@@ -11,7 +11,7 @@ require_once dirname(__DIR__) . '/helpers/ErrorHandler.php';
 ErrorHandler::init();
 
 try {
-    session_start();
+    if (session_status() === PHP_SESSION_NONE) session_start();
     require_once dirname(__DIR__) . '/config/config.php';
     require_once dirname(__DIR__) . '/helpers/Database.php';
     require_once dirname(__DIR__) . '/models/TemplateModel.php';