feat: add AI assistant with in-session copilot and standalone chat with RAG

Implements three-phase AI assistant feature:
- Phase 0: RAG infrastructure with pgvector embeddings, Voyage AI integration,
  tree chunking service, and semantic search over team's flow library
- Phase 1: In-session copilot panel during flow navigation with contextual
  AI help, current step awareness, and suggested related flows
- Phase 2: Standalone AI chat page with persistent conversation history,
  pin/delete, and configurable retention policies (account-level)

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

backend/app/core/config.py

@@ -84,6 +84,11 @@ class Settings(BaseSettings):
     AI_MODEL_GEMINI: str = "gemini-2.5-flash"
     AI_MODEL_ANTHROPIC: str = "claude-haiku-4-5-20251001"
 
+    # Embedding / RAG
+    VOYAGE_API_KEY: Optional[str] = None
+    EMBEDDING_MODEL: str = "voyage-3.5"
+    EMBEDDING_DIMENSIONS: int = 1024
+
     @property
     def ai_enabled(self) -> bool:
         """Check if any AI provider is configured."""