feat: add OpenRouter as an AI provider (AIIA-86)

Integrate OpenRouter via its OpenAI-compatible API so users can select
and use OpenRouter models alongside existing Anthropic/OpenAI/Ollama
providers. Adds provider to type system, DB enum, API validation,
buildModel switch, and settings UI.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

drizzle/0004_add_openrouter_provider.sql

@@ -0,0 +1,2 @@
+-- Add 'openrouter' to the api_key_provider enum
+ALTER TYPE "api_key_provider" ADD VALUE IF NOT EXISTS 'openrouter';