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

src/config.php

@@ -0,0 +1,55 @@
+<?php
+
+/**
+ * Configuration for Post-ERG thesis database
+ * Central location for database paths and environment settings
+ */
+
+// Database paths relative to repository root
+define('DB_ROOT', __DIR__ . '/..');
+
+// Test database (used in development)
+define('DB_TEST_PATH', DB_ROOT . '/database/test.db');
+
+// Production database (used on server)
+define('DB_PROD_PATH', DB_ROOT . '/database/posterg.db');
+
+/**
+ * Determine which database to use
+ * Checks environment variable DB_ENV, defaults to auto-detection
+ *
+ * Set DB_ENV in your environment:
+ * - export DB_ENV=test     # Force test database
+ * - export DB_ENV=prod     # Force production database
+ *
+ * Auto-detection logic:
+ * - If test.db exists, use it (development)
+ * - Otherwise use posterg.db (production)
+ */
+function getDatabasePath() {
+    // Allow explicit override via environment variable
+    $env = getenv('DB_ENV');
+
+    if ($env === 'test') {
+        return DB_TEST_PATH;
+    }
+
+    if ($env === 'prod') {
+        return DB_PROD_PATH;
+    }
+
+    // Auto-detect: prefer test database if it exists
+    if (file_exists(DB_TEST_PATH)) {
+        return DB_TEST_PATH;
+    }
+
+    // Default to production database
+    return DB_PROD_PATH;
+}
+
+/**
+ * Check if running in test/development mode
+ */
+function isTestMode() {
+    return getDatabasePath() === DB_TEST_PATH;
+}