feat: add OpenRouter as LLM and embedding provider

[oglenyaboss]

Summary

• Add OpenRouter as a first-class LLM and embedding provider, enabling access to 200+ models through a single OPENROUTER_API_KEY
• Both repowise init (doc generation) and repowise serve (chat/search) work with OpenRouter out of the box
• No new pip dependency — uses the existing openai package (OpenAI-compatible API)

LLM Provider

• OpenRouterProvider with generate() + stream_chat() support
• Default model: anthropic/claude-sonnet-4.6
• Sets recommended HTTP-Referer and X-Title headers for OpenRouter dashboard tracking
• Rate limits: 60 RPM / 200K TPM (conservative defaults, user-overridable)

Embedding Provider

• OpenRouterEmbedder for semantic search and chat RAG
• Default model: google/gemini-embedding-001 (768 dims)
• One API key covers both LLM and embeddings

Integration Points

• Registered in both LLM and embedding registries (lazy import)
• CLI auto-detection from OPENROUTER_API_KEY env var
• Interactive provider/embedder selection in repowise init and repowise serve
• Server provider catalog updated for web UI

Known Limitations

• Cost tracking disabled for OpenRouter: since it proxies 200+ models with varying prices, the fallback pricing would show inflated numbers. Users should check the OpenRouter dashboard for actual costs. A future PR could fetch real pricing via the /api/v1/models endpoint.
• Chat and embeddings not fully tested — LLM generation was tested end-to-end on a real 800+ file project using qwen/qwen3.6-plus via OpenRouter. Chat and embedding functionality was not integration-tested but follows the same OpenAI-compatible patterns as the existing OpenAI provider.

Test plan

• 13 unit tests for OpenRouterProvider (construction, generation, error mapping, headers)
• Registry test updated (builtin count 6 → 7, openrouter in list)
• Integration test added (skipped without OPENROUTER_API_KEY)
• Manual end-to-end test: repowise init with OpenRouter on a real project (328 wiki pages generated successfully)
• Chat via web UI with OpenRouter as active provider
• Embedding search with OpenRouterEmbedder