[Feature] Auto-load shipped fasttext model when present

[Feature] Auto-load shipped fasttext model when present

When no fasttext_model is configured, fall back to the model shipped at
$SHAREDIR/languages/fasttext_model.ftz: if the file is readable, load
it via the existing direct-load path; otherwise stay silent (debug
only) so stock installs without the model behave exactly as before.

This lets images that ship the model file drop the explicit
fasttext_model config override. The success path reuses
load_model_direct (the same code used for an explicit fasttext_model),
and the absent-file case produces no error and leaves the detector
reporting 'fasttext model is not loaded' as before.