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>
