misc: Refactor scan process by splitting single function

This change mainly refactors the `scan_platforms` function, moving part
of its logic to `_identify_platform`, `_identify_firmware`, and
`_identify_rom`.

The logic is simpler this way, each smaller function returns `ScanStats`
that can be merged by the caller, and it simplifies future performance
improvements.

backend/handler/database/platforms_handler.py

@@ -29,7 +29,7 @@ class DBPlatformsHandler(DBBaseHandler):
     @with_roms
     def add_platform(
         self, platform: Platform, query: Query = None, session: Session = None
-    ) -> Platform | None:
+    ) -> Platform:
         platform = session.merge(platform)
         session.flush()