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>

schema.sqlite.sql

@@ -0,0 +1,103 @@
+-- SQLite schema for Kabuki CMS / pics
+--
+-- Usage:
+--   sqlite3 data/pics.sqlite < schema.sqlite.sql
+--
+-- Config (add to config.php):
+--   define('DB_DRIVER', 'sqlite');
+--   define('DB_FILE', __DIR__ . '/data/pics.sqlite');
+
+CREATE TABLE IF NOT EXISTS users (
+	id_user INTEGER PRIMARY KEY,
+	first_name TEXT NOT NULL,
+	surname TEXT NOT NULL,
+	slug TEXT NOT NULL UNIQUE,
+	emailaddress TEXT NOT NULL UNIQUE,
+	password_hash TEXT NOT NULL,
+	creation_time INTEGER NOT NULL,
+	last_action_time INTEGER,
+	ip_address TEXT,
+	is_admin INTEGER NOT NULL DEFAULT 0,
+	reset_key TEXT,
+	reset_blocked_until INTEGER
+);
+
+CREATE TABLE IF NOT EXISTS assets (
+	id_asset INTEGER PRIMARY KEY,
+	id_user_uploaded INTEGER NOT NULL,
+	subdir TEXT NOT NULL,
+	filename TEXT NOT NULL,
+	title TEXT,
+	slug TEXT UNIQUE,
+	mimetype TEXT,
+	image_width INTEGER,
+	image_height INTEGER,
+	date_captured TEXT,
+	priority INTEGER DEFAULT 0
+);
+
+CREATE TABLE IF NOT EXISTS assets_meta (
+	id_asset INTEGER NOT NULL,
+	variable TEXT NOT NULL,
+	value TEXT,
+	PRIMARY KEY (id_asset, variable)
+);
+
+CREATE TABLE IF NOT EXISTS assets_thumbs (
+	id_asset INTEGER NOT NULL,
+	width INTEGER NOT NULL,
+	height INTEGER NOT NULL,
+	mode TEXT,
+	filename TEXT,
+	PRIMARY KEY (id_asset, width, height, mode)
+);
+
+CREATE TABLE IF NOT EXISTS tags (
+	id_tag INTEGER PRIMARY KEY,
+	id_parent INTEGER,
+	id_asset_thumb INTEGER,
+	id_user_owner INTEGER,
+	tag TEXT NOT NULL,
+	slug TEXT NOT NULL UNIQUE,
+	description TEXT,
+	kind TEXT NOT NULL DEFAULT 'Tag',
+	count INTEGER DEFAULT 0
+);
+
+CREATE TABLE IF NOT EXISTS assets_tags (
+	id_asset INTEGER NOT NULL,
+	id_tag INTEGER NOT NULL,
+	PRIMARY KEY (id_asset, id_tag)
+);
+
+CREATE TABLE IF NOT EXISTS posts_assets (
+	id_post INTEGER NOT NULL,
+	id_asset INTEGER NOT NULL,
+	PRIMARY KEY (id_post, id_asset)
+);
+
+CREATE TABLE IF NOT EXISTS posts_tags (
+	id_post INTEGER NOT NULL,
+	id_tag INTEGER NOT NULL,
+	PRIMARY KEY (id_post, id_tag)
+);
+
+CREATE TABLE IF NOT EXISTS settings (
+	id_user INTEGER NOT NULL,
+	variable TEXT NOT NULL,
+	value TEXT,
+	time_set TEXT DEFAULT CURRENT_TIMESTAMP,
+	PRIMARY KEY (id_user, variable)
+);
+
+CREATE TABLE IF NOT EXISTS log_errors (
+	id_entry INTEGER PRIMARY KEY,
+	id_user INTEGER,
+	message TEXT,
+	debug_info TEXT,
+	file TEXT,
+	line INTEGER,
+	request_uri TEXT,
+	time TEXT DEFAULT CURRENT_TIMESTAMP,
+	ip_address TEXT
+);