server: update model blob and manifest permissions

[LinasVidziunas]

This PR updates the permissions for the model blob and manifest files to 666, and directories to 777, before applying umask.

We're using Ollama in a multi-user environment and want to set up a shared models directory to prevent duplicate downloads. Currently, model files (644) and directories (755) restrict sharing, regardless of umask. With this change, setting a permissive umask (e.g., 000) allows all users to read/write shared models, while the default umask (022) keeps the existing behavior.

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