feat(fc3b): migration 0012 + AppSetting model

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/models/app_setting.py

@@ -0,0 +1,24 @@
+"""AppSetting — small key/value app config (extension API key, etc.).
+
+For settings that need DB persistence + runtime mutation. Use
+`ImportSettings` for the structured import-tuning settings; this
+table is for one-off scalar values keyed by string.
+"""
+
+from datetime import datetime
+
+from sqlalchemy import DateTime, String, Text, func
+from sqlalchemy.orm import Mapped, mapped_column
+
+from .base import Base
+
+
+class AppSetting(Base):
+    __tablename__ = "app_setting"
+
+    key: Mapped[str] = mapped_column(String(64), primary_key=True)
+    value: Mapped[str] = mapped_column(Text, nullable=False)
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False,
+        server_default=func.now(), onupdate=func.now(),
+    )