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/assistant_chat.py

@@ -0,0 +1,59 @@
+"""Pydantic schemas for standalone AI assistant chat."""
+from typing import Optional, Any
+from uuid import UUID
+from datetime import datetime
+from pydantic import BaseModel, Field
+
+from app.schemas.copilot import SuggestedFlow
+
+
+class ChatCreateRequest(BaseModel):
+    """Empty body — creates a new blank conversation."""
+    pass
+
+
+class ChatMessageRequest(BaseModel):
+    message: str = Field(..., min_length=1, max_length=8000)
+
+
+class ChatMessageResponse(BaseModel):
+    content: str
+    suggested_flows: list[SuggestedFlow] = []
+
+
+class ChatListResponse(BaseModel):
+    id: UUID
+    title: str
+    message_count: int
+    pinned: bool
+    created_at: datetime
+    updated_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
+class ChatDetailResponse(BaseModel):
+    id: UUID
+    title: str
+    messages: list[dict[str, Any]]
+    message_count: int
+    pinned: bool
+    created_at: datetime
+    updated_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
+class ChatUpdateRequest(BaseModel):
+    title: Optional[str] = Field(None, min_length=1, max_length=255)
+    pinned: Optional[bool] = None
+
+
+class RetentionSettingsResponse(BaseModel):
+    chat_retention_days: Optional[int]
+    chat_retention_max_count: Optional[int]
+
+
+class RetentionSettingsUpdate(BaseModel):
+    chat_retention_days: Optional[int] = Field(None, ge=1, le=365)
+    chat_retention_max_count: Optional[int] = Field(None, ge=10, le=10000)