Add SQLite support as alternative database backend

Support config-driven choice between MySQL and SQLite via DB_DRIVER
constant, defaulting to MySQL for backward compatibility. All SQL
adaptation lives in Database.php (UDFs + query rewriting), so model
files need no changes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

SQLite: remove FK constraints, revert 0→null sentinel changes

The SQLite schema had FOREIGN KEY constraints that don't exist in the
MySQL schema. These forced a cascade of 0→null changes to satisfy FK
enforcement. Removing them keeps the two backends behaviorally consistent
and minimises the diff. Real SQLite compat fixes (UDFs, query rewriting,
rowCount→count, Router fixes, EditAlbum guard) are preserved.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

seed.php

@@ -0,0 +1,58 @@
+<?php
+/*****************************************************************************
+ * seed.php
+ * Seeds a fresh database with an admin user and root album.
+ *
+ * Usage: php seed.php
+ *****************************************************************************/
+
+require_once __DIR__ . '/config.php';
+require_once __DIR__ . '/vendor/autoload.php';
+
+if (!defined('DB_DRIVER') || DB_DRIVER !== 'sqlite')
+{
+	echo "Error: seed.php currently only supports SQLite.\n";
+	echo "Set DB_DRIVER to 'sqlite' in config.php.\n";
+	exit(1);
+}
+
+if (!file_exists(DB_FILE))
+{
+	echo "Error: database file not found at " . DB_FILE . "\n";
+	echo "Create it first: sqlite3 " . DB_FILE . " < schema.sqlite.sql\n";
+	exit(1);
+}
+
+$db = new Database('sqlite', ['file' => DB_FILE]);
+Registry::set('db', $db);
+
+// Create admin user.
+$password = 'admin';
+$hash = password_hash($password, PASSWORD_DEFAULT);
+
+$db->insert('insert', 'users', [], [
+	'first_name' => 'Admin',
+	'surname' => 'User',
+	'slug' => 'admin',
+	'emailaddress' => 'admin@localhost',
+	'password_hash' => $hash,
+	'creation_time' => time(),
+	'ip_address' => '',
+	'is_admin' => 1,
+	'reset_key' => '',
+]);
+
+echo "Created admin user (admin@localhost / admin)\n";
+
+// Create root album (id_tag = 1).
+$db->insert('insert', 'tags', [], [
+	'id_parent' => 0,
+	'tag' => 'Albums',
+	'slug' => 'albums',
+	'kind' => 'Album',
+	'description' => '',
+	'count' => 0,
+]);
+
+echo "Created root album (id_tag = 1)\n";
+echo "\nDone. You can now log in at the web UI.\n";