ref(backend): Centralize deps and add mypy

Extract reusable API dependencies and activity logging helpers.\nAdd mypy configuration and dev dependency for type checking.\n\nCo-Authored-By: Claude <noreply@anthropic.com>

backend/app/services/activity_log.py

@@ -0,0 +1,25 @@
+from __future__ import annotations
+
+from uuid import UUID
+
+from sqlmodel import Session
+
+from app.models.activity_events import ActivityEvent
+
+
+def record_activity(
+    session: Session,
+    *,
+    event_type: str,
+    message: str,
+    agent_id: UUID | None = None,
+    task_id: UUID | None = None,
+) -> ActivityEvent:
+    event = ActivityEvent(
+        event_type=event_type,
+        message=message,
+        agent_id=agent_id,
+        task_id=task_id,
+    )
+    session.add(event)
+    return event