Add OpenRouter embeddings support

Implements OpenRouter as an embedding provider option, enabling access to multiple
embedding models (OpenAI, Google Gemini, Qwen3, Mistral) through a single API key.

Backend changes:
- Add validate_openrouter_api_key() for API key validation (sk-or-v1- format)
- Add OpenRouterErrorAdapter for error sanitization
- Add openrouter to valid providers in llm_provider_service
- Create openrouter_discovery_service with hardcoded model list
- Create /api/openrouter/models endpoint for model discovery
- Register OpenRouter router in FastAPI main app

Frontend changes:
- Create openrouterService.ts for model discovery API client
- Add OpenRouter to RAGSettings.tsx provider options
- Configure default models with provider prefix (openai/text-embedding-3-small)
- Add OpenRouter to embedding-capable providers list

Documentation:
- Update .env.example with OPENROUTER_API_KEY documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.env.example

@@ -125,7 +125,8 @@ PROD=false
 # Run the credentials_setup.sql file in your Supabase SQL editor to set up the credentials table.
 # Then use the Settings page in the web UI to manage:
 # - OPENAI_API_KEY (encrypted)
-# - MODEL_CHOICE 
+# - OPENROUTER_API_KEY (encrypted, format: sk-or-v1-..., get from https://openrouter.ai/keys)
+# - MODEL_CHOICE
 # - TRANSPORT settings
 # - RAG strategy flags (USE_CONTEXTUAL_EMBEDDINGS, USE_HYBRID_SEARCH, etc.)
 # - Crawler settings: