feat: add OpenAI-first provider selection with OpenRouter fallback

[mjalalimanesh]

Summary

This PR adds provider selection that preserves original OpenAI behavior and uses OpenRouter only as fallback.

Behavior

• If OPENAI_API_KEY is set: Dash uses OpenAI model + OpenAI embedding path (original behavior).
• Else if OPENROUTER_API_KEY is set: Dash uses OpenRouter model + OpenRouter-backed embedding path.
• If both are set: OpenAI takes priority.
• If neither is set: current failure behavior is unchanged.

Changes

• dash/agents.py
  • Added provider selection logic (OpenAI first, OpenRouter fallback).
  • Added OpenRouter model/embedding configuration for fallback path only.
• example.env
  • Documented key priority and fallback behavior.
• README.md
  • Updated env docs and quick-start note for key priority.

Scope control

Included only runtime/docs for provider fallback.

Not included:

• .gitlab-ci.yml
• compose.yaml
• deployment/infrastructure changes
• analytics-db related changes

Validation

• python -m compileall dash db app
• Manual sanity checks for key matrix:
  • OpenAI only -> OpenAI path
  • OpenRouter only -> OpenRouter path
  • both -> OpenAI path