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

public/memoire.php

@@ -3,7 +3,7 @@
 require_once __DIR__ . '/../config/bootstrap.php';
 
 // Load required libraries and classes
-require_once APP_ROOT . '/lib/Database.php';
+require_once APP_ROOT . '/src/Database.php';
 
 // Check if an id parameter is provided in the URL
 if (isset($_GET['id'])) {
@@ -29,7 +29,7 @@ if (isset($_GET['id'])) {
 }
 
 // Include the header template
-include APP_ROOT . '/includes/header.php'; ?>
+include APP_ROOT . '/public/includes/header.php'; ?>
 <main>
     <div class="item">
         <div class="card-content">
@@ -151,4 +151,4 @@ include APP_ROOT . '/includes/header.php'; ?>
 </main>
 
 <!-- Include the footer template -->
-<?php include APP_ROOT . '/includes/footer.php'; ?>
+<?php include APP_ROOT . '/public/includes/footer.php'; ?>