Add RAG embedding and AI chat feature

Implement end-to-end RAG pipeline: AST-aware code chunking, LiteLLM
embedding generation, MongoDB vector storage with brute-force cosine
similarity fallback for self-hosted instances, and a chat API with
RAG-augmented responses. Add dedicated /chat/:repo_id dashboard page
with embedding build controls, message history, and source reference
cards.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

compliance-core/src/config.rs

@@ -8,6 +8,7 @@ pub struct AgentConfig {
     pub litellm_url: String,
     pub litellm_api_key: SecretString,
     pub litellm_model: String,
+    pub litellm_embed_model: String,
     pub github_token: Option<SecretString>,
     pub github_webhook_secret: Option<SecretString>,
     pub gitlab_url: Option<String>,