feat(date-memories): add DateMemory domain model

app/src/main/java/app/closer/domain/model/DateMemory.kt

@@ -0,0 +1,18 @@
+package app.closer.domain.model
+
+/**
+ * A completed date the couple logged, for the Date Replay timeline.
+ *
+ * Stored at `couples/{coupleId}/date_history/{id}` as PLAINTEXT (app content / coarse metadata — the
+ * date-idea title + category, not the partners' private words; consistent with the metadata stance).
+ * The private post-date words live separately in [DateReflection] (E2EE). Doc id = the source matchId so
+ * "mark done" is idempotent.
+ */
+data class DateMemory(
+    val id: String = "",
+    val dateIdeaId: String = "",
+    val title: String = "",
+    val category: String = "",
+    val completedAt: Long = 0L,
+    val addedBy: String = ""
+)