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

tests/Unit/DatabaseTest.php

@@ -4,7 +4,7 @@
  * Tests basic database connectivity and query functionality
  */
 
-require_once __DIR__ . '/../../lib/Database.php';
+require_once __DIR__ . '/../../src/Database.php';
 
 echo "Database Connection Test\n";
 echo "========================\n\n";

tests/Unit/RateLimitTest.php

@@ -4,7 +4,7 @@
  * Tests rate limiting functionality
  */
 
-require_once __DIR__ . '/../../lib/RateLimit.php';
+require_once __DIR__ . '/../../src/RateLimit.php';
 
 echo "Rate Limit Test\n";
 echo "===============\n\n";