System-scoped DAT import refactor and metadata matching improvements

Refactored DAT import workflow so No-Intro and TOSEC imports are tied to a selected local main_app_system instead of global table replacement.
Updated UI import flow to prompt for target system before DAT import.
Added schema support for per-system DAT scoping (app_system_id) and DAT import tracking (dat_import_history with source/version/date/count).
Updated No-Intro/TOSEC import logic to replace rows only for the selected system and upsert last-import metadata.
Switched game metadata matching to use main_app_file_hash.file_md5_hash (via container_file_id) instead of container hash.
Updated game list/detail lookups to use file-level hash matching, including case-insensitive MD5 comparison.
Improved TOSEC/No-Intro lookup accuracy and kept read paths non-destructive (schema writes only during migration/import paths).
Added/updated indexing and normalization hooks to support system-scoped matching performance and backward compatibility.