refactor: reorganize to standard PHP structure

- Moved /lib → /src (PHP source code)
- Moved /includes → /public/includes (main site templates)
- Admin section remains self-contained in /public/admin with its own /inc
- Updated all require/include paths across codebase
- Updated config/bootstrap.php, justfile, tests, docs
- All tests passing ✅

Structure now follows PHP best practices:
  /config      - Configuration files
  /database    - SQLite database + schema
  /docs        - Documentation (intact)
  /nginx       - Server config (intact)
  /public      - Web-accessible files (entry point)
    /admin     - Self-contained admin interface
    /assets    - CSS, fonts, icons
    /includes  - Main site templates (header/footer)
  /scripts     - Deployment scripts (intact)
  /src         - PHP source classes (Database, AdminAuth, RateLimit)
  /tests       - Test suites

config/bootstrap.php

@@ -25,7 +25,7 @@ if (php_sapi_name() === 'cli-server') {
 
 // Simple helper function for including templates
 function include_template($name) {
-    $path = APP_ROOT . '/includes/' . $name;
+    $path = APP_ROOT . '/public/includes/' . $name;
     if (file_exists($path)) {
         include $path;
     }