feat(core): bind request services and implement registered tasks

Wire plugin services through Bind/InjectFrom and AppContext so HTTP and
workers resolve dependencies after Apply. Register TaskHandler objects
with persisted results, and implement send_email_code, mail:send,
cleanup_inactive_users, and dispatch_bot_msg.

backend/plugins/domain/domain_test.go

@@ -236,7 +236,7 @@ func TestUserPlugin(t *testing.T) {
 	assert.Len(t, list, 1)
 	assert.Equal(t, "bob", list[0].Username)
 
-	// 9. Tasks & Schedules
+	// 9. Tasks
 	taskDef, ok := ctx.Tasks().Get("user:send_email_code")
 	require.True(t, ok)
 	assert.Equal(t, 3, taskDef.Retry)