feat: AppSetting model and migration 0004

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

fablednetmon/models/settings.py

@@ -0,0 +1,18 @@
+from __future__ import annotations
+from datetime import datetime, timezone
+from sqlalchemy import DateTime, String, Text
+from sqlalchemy.orm import Mapped, mapped_column
+from fablednetmon.models.base import Base
+
+
+class AppSetting(Base):
+    __tablename__ = "app_settings"
+
+    key: Mapped[str] = mapped_column(String(128), primary_key=True)
+    value_json: Mapped[str] = mapped_column(Text, nullable=False)
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        nullable=False,
+        default=lambda: datetime.now(timezone.utc),
+        onupdate=lambda: datetime.now(timezone.utc),
+    )