feat: activity log model + API (audit trail)

app/models/activity.py

@@ -0,0 +1,15 @@
+from sqlalchemy import Column, Integer, String, Text, DateTime, ForeignKey
+from sqlalchemy.sql import func
+from app.core.config import Base
+
+
+class ActivityLog(Base):
+    __tablename__ = "activity_logs"
+
+    id = Column(Integer, primary_key=True, index=True)
+    action = Column(String(50), nullable=False)  # e.g. "issue.created", "comment.added"
+    entity_type = Column(String(50), nullable=False)  # "issue", "project", "comment"
+    entity_id = Column(Integer, nullable=False)
+    user_id = Column(Integer, ForeignKey("users.id"), nullable=True)
+    details = Column(Text, nullable=True)  # JSON string
+    created_at = Column(DateTime(timezone=True), server_default=func.now())