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/account.py

@@ -1,7 +1,7 @@
 import uuid
 from datetime import datetime, timezone
 from typing import Optional, TYPE_CHECKING
-from sqlalchemy import String, DateTime, ForeignKey, Boolean
+from sqlalchemy import String, DateTime, ForeignKey, Boolean, Integer
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 from sqlalchemy.dialects.postgresql import UUID
 from app.core.database import Base
@@ -35,6 +35,14 @@ class Account(Base):
         comment="Policy: engineers can create public shares. Only affects NEW shares (grandfathered)."
     )
 
+    # Chat retention settings
+    chat_retention_days: Mapped[Optional[int]] = mapped_column(
+        Integer, nullable=True, default=90, server_default="90"
+    )
+    chat_retention_max_count: Mapped[Optional[int]] = mapped_column(
+        Integer, nullable=True, default=100, server_default="100"
+    )
+
     # Relationships
     owner: Mapped["User"] = relationship("User", foreign_keys=[owner_id], back_populates="owned_account")
     users: Mapped[list["User"]] = relationship("User", foreign_keys="[User.account_id]", back_populates="account")