feat: add OpenAI-compatible embedding provider

[HosseinShahabadi]

Summary

Adds support for any OpenAI-compatible embeddings endpoint as an
alternative to fastembed. This allows users to use OpenAI, OpenRouter,
Azure OpenAI, Together AI, or any other compatible provider without
needing to run a local model.

Changes

• embeddings/types.py — added OPENAI to EmbeddingProviderType enum
• embeddings/openai.py — new provider implementing EmbeddingProvider
• embeddings/factory.py — added openai branch to create_embedding_provider
• settings.py — added OPENAI_API_KEY, OPENAI_BASE_URL, OPENAI_VECTOR_SIZE fields
• README.md — documented new env vars and usage examples

New environment variables

Variable	Description	Default
OPENAI_API_KEY	API key for the embedding service	None
OPENAI_BASE_URL	Base URL of the endpoint	https://api.openai.com/v1
OPENAI_VECTOR_SIZE	Vector dimensions (auto-detected if omitted)	None

Example (OpenRouter)

EMBEDDING_PROVIDER="openai" \
OPENAI_API_KEY="sk-or-v1-..." \
OPENAI_BASE_URL="https://openrouter.ai/api/v1" \
EMBEDDING_MODEL="qwen/qwen3-embedding-4b" \
OPENAI_VECTOR_SIZE=2560 \
uvx mcp-server-qdrant

Testing

Tested locally with OpenRouter + qwen/qwen3-embedding-4b against
a local Qdrant instance. Both qdrant-store and qdrant-find tools
work correctly via MCP inspector.

[castlenthesky]

@HosseinShahabadi, letting you know I've opened #187 as a draft for the same feature. Yours is the newest of three open OpenAI-compatible PRs (#92, #111, #158) and it felt wrong to add a fourth without saying anything.

#158 has gone stale against master and hasn't been reviewed. The pattern I see is that this repo is short on review bandwidth, and the one piece of guidance we've actually got from a maintainer (@kacperlukawski in #55, asking for "just OpenAI embeddings, but nothing else") hasn't been met by any of the three, since all of them ship refactors alongside the feature.

So mine is additive only: 26 lines touched in existing files, one new provider, FastEmbed left alone. If you'd rather push #158 across the line yourself, I'll close mine and help with the rebase. I just want the feature to exist.