Refactor backend to SQLModel; reset schema; add Company OS endpoints

backend/app/models/activity.py

@@ -0,0 +1,19 @@
+from __future__ import annotations
+
+from datetime import datetime
+
+from sqlmodel import Field, SQLModel
+
+
+class Activity(SQLModel, table=True):
+    __tablename__ = "activities"
+
+    id: int | None = Field(default=None, primary_key=True)
+    actor_employee_id: int | None = Field(default=None, foreign_key="employees.id")
+
+    entity_type: str
+    entity_id: int | None = None
+    verb: str
+
+    payload_json: str | None = None
+    created_at: datetime = Field(default_factory=datetime.utcnow)