feat(example): add OpenAI-compatible embeddings endpoint

[abetlen]

Add OpenAI-compatible embeddings support to the server example.

• Adds POST /v1/embeddings with string, token, batched, float, base64, and dimensions support.
• Auto-detects pooled embedding GGUFs from model metadata while preserving explicit embedding overrides.
• Runs embedding requests when the scheduler is idle and adds a small BGE config plus README docs.