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/schemas/copilot.py

@@ -0,0 +1,44 @@
+"""Pydantic schemas for the in-session copilot."""
+from typing import Optional, Any
+from uuid import UUID
+from datetime import datetime
+from pydantic import BaseModel, Field
+
+
+class SuggestedFlow(BaseModel):
+    tree_id: UUID
+    tree_name: str
+    tree_type: str
+    relevance_snippet: str
+
+
+class CopilotStartRequest(BaseModel):
+    tree_id: UUID
+    session_id: Optional[UUID] = None
+    current_node_id: Optional[str] = None
+
+
+class CopilotStartResponse(BaseModel):
+    conversation_id: UUID
+    greeting: str
+
+
+class CopilotMessageRequest(BaseModel):
+    message: str = Field(..., min_length=1, max_length=4000)
+    current_node_id: Optional[str] = None
+
+
+class CopilotMessageResponse(BaseModel):
+    content: str
+    suggested_flows: list[SuggestedFlow] = []
+
+
+class CopilotConversationResponse(BaseModel):
+    id: UUID
+    tree_id: UUID
+    messages: list[dict[str, Any]]
+    current_node_id: Optional[str] = None
+    message_count: int
+    created_at: datetime
+
+    model_config = {"from_attributes": True}