feat: add gemma4:12b Ollama model

[TylersHub]

Summary

Adds gemma4:12b to the supported Ollama models in providers.json.

Motivation

gemma4:12b is a distinct Ollama model variant, not an alias for the
current gemma4:latest model. Registering it allows users to explicitly
select the 12B model through DEFAULT_MODEL.

Testing

• Pulled gemma4:12b through Ollama
• Set DEFAULT_MODEL=gemma4:12b
• Validated providers.json with python -m json.tool providers.json
• Ran the complete scoring pipeline with two sample resumes
• Confirmed section extraction, structured output, and evaluation completed

Cross-provider testing was not performed because this change only registers
an Ollama-specific model and does not alter shared provider logic or prompts.

Closes #380