Implement initial scan tracking for one-time setup

- Add SystemSettings model to track setup completion status
- Create SystemSettingsService for managing setup flags
- Modify fastapi_app startup to check and set initial_scan_completed flag
- Anime folder scanning now only runs on first startup
- Update DATABASE.md with new system_settings table documentation
- Add unit test for SystemSettingsService functionality

This ensures expensive one-time operations like scanning the entire anime
directory only occur during initial setup, not on every application restart.

src/server/database/models.py

@@ -543,3 +543,52 @@ class UserSession(Base, TimestampMixin):
     def revoke(self) -> None:
         """Revoke this session."""
         self.is_active = False
+
+
+class SystemSettings(Base, TimestampMixin):
+    """SQLAlchemy model for system-wide settings and state.
+    
+    Stores application-level configuration and state flags that persist
+    across restarts. Used to track initialization status and setup completion.
+    
+    Attributes:
+        id: Primary key (single row expected)
+        initial_scan_completed: Whether the initial anime folder scan has been completed
+        initial_nfo_scan_completed: Whether the initial NFO scan has been completed
+        initial_media_scan_completed: Whether the initial media scan has been completed
+        last_scan_timestamp: Timestamp of the last completed scan
+        created_at: Creation timestamp (from TimestampMixin)
+        updated_at: Last update timestamp (from TimestampMixin)
+    """
+    __tablename__ = "system_settings"
+    
+    # Primary key (only one row should exist)
+    id: Mapped[int] = mapped_column(
+        Integer, primary_key=True, autoincrement=True
+    )
+    
+    # Setup/initialization tracking
+    initial_scan_completed: Mapped[bool] = mapped_column(
+        Boolean, nullable=False, default=False, server_default="0",
+        doc="Whether the initial anime folder scan has been completed"
+    )
+    initial_nfo_scan_completed: Mapped[bool] = mapped_column(
+        Boolean, nullable=False, default=False, server_default="0",
+        doc="Whether the initial NFO scan has been completed"
+    )
+    initial_media_scan_completed: Mapped[bool] = mapped_column(
+        Boolean, nullable=False, default=False, server_default="0",
+        doc="Whether the initial media scan has been completed"
+    )
+    last_scan_timestamp: Mapped[Optional[datetime]] = mapped_column(
+        DateTime(timezone=True), nullable=True,
+        doc="Timestamp of the last completed scan"
+    )
+    
+    def __repr__(self) -> str:
+        return (
+            f"<SystemSettings(id={self.id}, "
+            f"initial_scan_completed={self.initial_scan_completed}, "
+            f"initial_nfo_scan_completed={self.initial_nfo_scan_completed}, "
+            f"initial_media_scan_completed={self.initial_media_scan_completed})>"
+        )