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/models/copilot_conversation.py

@@ -0,0 +1,69 @@
+"""Copilot conversation model for in-session AI assistant.
+
+Tracks conversation state during flow navigation with contextual AI help.
+"""
+import uuid
+from datetime import datetime, timezone
+from typing import Optional, Any
+
+from sqlalchemy import String, DateTime, ForeignKey, Integer
+from sqlalchemy.orm import Mapped, mapped_column
+from sqlalchemy.dialects.postgresql import UUID, JSONB
+
+from app.core.database import Base
+
+
+class CopilotConversation(Base):
+    __tablename__ = "copilot_conversations"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    user_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("users.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    account_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    session_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("sessions.id", ondelete="SET NULL"),
+        nullable=True,
+    )
+    tree_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("trees.id", ondelete="CASCADE"),
+        nullable=False,
+    )
+    messages: Mapped[list[dict[str, Any]]] = mapped_column(
+        JSONB, nullable=False, default=list
+    )
+    current_node_id: Mapped[Optional[str]] = mapped_column(
+        String(100), nullable=True
+    )
+    message_count: Mapped[int] = mapped_column(
+        Integer, nullable=False, default=0
+    )
+    total_input_tokens: Mapped[int] = mapped_column(
+        Integer, nullable=False, default=0
+    )
+    total_output_tokens: Mapped[int] = mapped_column(
+        Integer, nullable=False, default=0
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc),
+        onupdate=lambda: datetime.now(timezone.utc),
+    )
+    expires_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False
+    )