Add OpenRouter multi-model support and Voyage AI semantic search

OpenRouter: extends the LLM provider abstraction to support GPT-4o,
GPT-4 Turbo, Mistral Large, Llama 3.3 70B, and Gemma 3 27B via a
single OpenRouter API key. Follows the same provider pattern as Claude
and Gemini - new openrouter.ts adapter, model routing by prefix, per-user
API key stored in user_profiles, and full UI integration (model toggle,
availability checks, account settings key field).

Semantic search: adds Voyage AI embeddings (voyage-law-2, optimised for
legal documents) to index document chunks at upload time (fire-and-forget,
controlled by VOYAGE_API_KEY env var). Chunks are stored in a new
document_chunks pgvector table. A /search/semantic endpoint and a
semantic_search LLM tool enable concept-level retrieval. A new
SemanticSearchWidget in ProjectPage exposes document search by meaning.

https://claude.ai/code/session_01NJxa5NasJ9qhaMkzEyqr2u