feat: split initial social schema migration chain

replace monolithic migration with ordered scripts, include profiles/sessions in migration, and verify full downgrade/upgrade cycle for clean Supabase bootstrap

backend/src/models/agent_chat_session.py

@@ -8,7 +8,6 @@ from enum import Enum
 from sqlalchemy import (
     DateTime,
     Enum as SqlEnum,
-    ForeignKey,
     Integer,
     Numeric,
     String,
@@ -28,18 +27,32 @@ class AgentChatSessionStatus(str, Enum):
     FAILED = "failed"
 
 
+class SessionType(str, Enum):
+    CHAT = "chat"
+    AUTOMATION = "automation"
+
+
 class AgentChatSession(TimestampMixin, SoftDeleteMixin, Base):
     __tablename__: str = "sessions"
+    __table_args__ = {"extend_existing": True}
 
     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("auth.users.id", ondelete="CASCADE"),
         nullable=False,
         index=True,
     )
+    session_type: Mapped[SessionType] = mapped_column(
+        String(20),
+        nullable=False,
+        default=SessionType.CHAT,
+    )
+    job_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True),
+        nullable=True,
+    )
     title: Mapped[str | None] = mapped_column(String(255), nullable=True)
     status: Mapped[AgentChatSessionStatus] = mapped_column(
         SqlEnum(