Add static Context (Singleton) to DbBase

ActiveRecord pattern: expose a shared DbContext on DbBase so any
entity subclass can reach it without passing context through parameters.
The instance-level DbContext back-reference still works; the static one
provides a class-level global fallback.

src/Before/DbBase.cs

@@ -11,6 +11,21 @@ namespace Before;
 /// </summary>
 public class DbBase
 {
+    // -----------------------------------------------------------------
+    // Static singleton: an ActiveRecord-style shared context that any
+    // entity can reach without passing it through method parameters.
+    // -----------------------------------------------------------------
+    /// <summary>
+    /// Shared (singleton) <see cref="DbContext" /> accessible from every
+    /// entity via its base type.  Set once at application startup so that
+    /// entity methods can call <c>DbBase.Context!</c> instead of carrying
+    /// a context reference.
+    ///
+    /// This is yet another leak: domain objects depend on the persistence
+    /// layer at the *type* level, not just the instance level.
+    /// </summary>
+    public static DbContext? Context { get; set; }
+
     /// <summary>
     /// Primary key. Fresh (unsaved) entities have <see cref="Guid.Empty"/>;
     /// <see cref="DbContext.Save"/> assigns a real id, faking EF's identity