model: fix case-insensitive suffix matching and skip .bak files in ListFilesInModelPath

[pos-ei-don]

Problem

When huggingface_hub downloads models into LocalAIs models directory, it creates a CACHEDIR.TAG file. This file shows up in /v1/models because the suffix skip list contains .tag (lowercase), but strings.HasSuffix is case-sensitive — CACHEDIR.TAG (uppercase) is therefore not matched.

Additionally, backup files like model.yaml.bak-pre-gpumem072 appear in /v1/models. These are created e.g. when LocalAI backs up YAML configs before modifying them.

Fix

1. Case-insensitive suffix matching: The filename is lowercased once with strings.ToLower before comparing against entries in knownModelsNameSuffixToSkip. This makes .tag match CACHEDIR.TAG and similar variants.
2. .bak detection via contains-check: Backup files with suffixes like .bak-pre-gpumem072 do not end exactly in .bak, so an additional strings.Contains(lowerName, ".bak") check is added.
3. Cleanup: The duplicate .MD entry is removed (now redundant via case-insensitive .md matching). .DS_Store is replaced by lowercased .ds_store.

Reproduction

$ curl http://localhost:8080/v1/models | jq '.data[].id'
"CACHEDIR.TAG"
"coder.yaml.bak-pre-gpumem072"
"gemma4-12b.yaml.bak-pre-gpulayers"
...

Affects any installation that uses huggingface_hub for model downloads or has YAML backup files in the models directory.