feat(agent): session deletion anonymization for iOS compliance

Replace soft-delete with anonymize + hard-delete to meet iOS App Store
data retention requirements. Non-PII fields are preserved in
anonymous_session_snapshots for analytics.

- Add anonymous_session_snapshots table and ORM model
- Implement anonymizer to extract non-PII fields before deletion
- Remove points_ledger.biz_id FK constraint (snapshot-style reference)
- Preserve transaction history while allowing session deletion
- Add 14 unit tests + 1 integration test

backend/src/models/__init__.py

@@ -2,6 +2,7 @@ from __future__ import annotations
 
 from .agent_chat_message import AgentChatMessage
 from .agent_chat_session import AgentChatSession
+from .anonymous_session_snapshot import AnonymousSessionSnapshot
 from .auth_user import AuthUser
 from .invite_code import InviteCode
 from .llm import Llm
@@ -18,6 +19,7 @@ from .user_points import UserPoints
 __all__ = [
     "AgentChatMessage",
     "AgentChatSession",
+    "AnonymousSessionSnapshot",
     "AuthUser",
     "InviteCode",
     "Llm",

backend/src/models/anonymous_session_snapshot.py

@@ -0,0 +1,46 @@
+from __future__ import annotations
+
+from datetime import datetime
+from decimal import Decimal
+import uuid
+
+from sqlalchemy import Boolean, DateTime, Integer, Numeric, String, Text
+from sqlalchemy.dialects.postgresql import ARRAY, UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from core.db.base import Base
+
+__all__ = ["AnonymousSessionSnapshot"]
+
+
+class AnonymousSessionSnapshot(Base):
+    __tablename__: str = "anonymous_session_snapshots"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    anonymous_id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), nullable=False)
+    session_type: Mapped[str] = mapped_column(String(20), nullable=False)
+    message_count: Mapped[int | None] = mapped_column(Integer, nullable=True)
+    status: Mapped[str | None] = mapped_column(String(20), nullable=True)
+    question_type: Mapped[str | None] = mapped_column(String(50), nullable=True)
+    tool_name: Mapped[str | None] = mapped_column(String(100), nullable=True)
+    gua_name: Mapped[str | None] = mapped_column(String(50), nullable=True)
+    gua_name_hant: Mapped[str | None] = mapped_column(String(50), nullable=True)
+    target_gua_name: Mapped[str | None] = mapped_column(String(50), nullable=True)
+    has_changing_yao: Mapped[bool | None] = mapped_column(Boolean, nullable=True)
+    sign_level: Mapped[str | None] = mapped_column(String(20), nullable=True)
+    keywords: Mapped[list[str] | None] = mapped_column(ARRAY(Text()), nullable=True)
+    model_code: Mapped[str | None] = mapped_column(String(50), nullable=True)
+    total_tokens: Mapped[int | None] = mapped_column(Integer, nullable=True)
+    total_cost: Mapped[Decimal | None] = mapped_column(Numeric(12, 6), nullable=True)
+    total_latency_ms: Mapped[int | None] = mapped_column(Integer, nullable=True)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False
+    )
+    last_activity_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+    anonymized_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False
+    )