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

docs/DEVELOPMENT_GUIDE.md

@@ -130,7 +130,7 @@ just fixtures
 ### Create a New Page
 
 1. Create `newpage.php` in root
-2. Add `require_once __DIR__ . '/lib/Database.php';`
+2. Add `require_once __DIR__ . '/src/Database.php';`
 3. Include header: `include 'inc/header.php';`
 4. Add your content
 5. Include footer: `include 'inc/footer.php';`
@@ -138,7 +138,7 @@ just fixtures
 Example:
 ```php
 <?php
-require_once __DIR__ . '/lib/Database.php';
+require_once __DIR__ . '/src/Database.php';
 include 'inc/header.php';
 ?>
 
@@ -154,7 +154,7 @@ include 'inc/header.php';
 
 ### Add a Database Function
 
-1. Edit `lib/Database.php`
+1. Edit `src/Database.php`
 2. Add your method to the `Database` class
 3. Write a test in `tests/Unit/DatabaseTest.php`
 4. Run tests: `just test-unit`
@@ -215,7 +215,7 @@ See `tests/README.md` for complete testing guide.
 **Quick example:**
 ```php
 <?php
-require_once __DIR__ . '/../../lib/Database.php';
+require_once __DIR__ . '/../../src/Database.php';
 
 echo "My Test\n";
 echo "=======\n\n";
@@ -243,7 +243,7 @@ just deploy
 This deploys:
 - Public site (root PHP files)
 - Admin panel (`admin/`)
-- Shared libraries (`lib/`)
+- Shared libraries (`src/`)
 
 **Note:** `vendor/` is automatically excluded from deployment.