feat(backup): add ConversationCache Room DB (entity, DAO, database)

app/src/main/java/app/closer/data/local/ConversationCacheDao.kt

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+package app.closer.data.local
+
+import androidx.room.Dao
+import androidx.room.Insert
+import androidx.room.OnConflictStrategy
+import androidx.room.Query
+import app.closer.data.local.entity.ConversationCacheEntity
+
+@Dao
+interface ConversationCacheDao {
+
+    /** Idempotent restore: re-restoring the same message id replaces (latest-wins). */
+    @Insert(onConflict = OnConflictStrategy.REPLACE)
+    suspend fun upsertAll(rows: List<ConversationCacheEntity>)
+
+    @Query("SELECT * FROM conversation_cache WHERE conversationId = :conversationId ORDER BY createdAt ASC")
+    suspend fun forConversation(conversationId: String): List<ConversationCacheEntity>
+
+    @Query("SELECT COUNT(*) FROM conversation_cache")
+    suspend fun count(): Int
+
+    @Query("DELETE FROM conversation_cache")
+    suspend fun clear()
+}

app/src/main/java/app/closer/data/local/ConversationCacheDatabase.kt

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+package app.closer.data.local
+
+import androidx.room.Database
+import androidx.room.RoomDatabase
+import app.closer.data.local.entity.ConversationCacheEntity
+
+/**
+ * Separate Room DB for the restored conversation cache — kept apart from the asset-backed [AppDatabase]
+ * (the prepopulated question bank) so backup/restore never risks that DB's asset schema hash.
+ */
+@Database(entities = [ConversationCacheEntity::class], version = 1, exportSchema = false)
+abstract class ConversationCacheDatabase : RoomDatabase() {
+    abstract fun conversationCacheDao(): ConversationCacheDao
+}

app/src/main/java/app/closer/data/local/entity/ConversationCacheEntity.kt

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+package app.closer.data.local.entity
+
+import androidx.room.Entity
+import androidx.room.PrimaryKey
+
+/**
+ * A restored/backed-up message in the local durable conversation cache. Keyed by the Firestore
+ * message id so restore is idempotent (upsert = latest-wins on re-restore). `encText` keeps the
+ * `enc:v1:` body verbatim; `reactionsJson` is a small JSON object of uid→emoji. This store is the
+ * source of truth for restored history and the foundation for the later Option-B switch.
+ */
+@Entity(tableName = "conversation_cache")
+data class ConversationCacheEntity(
+    @PrimaryKey val messageId: String,
+    val conversationId: String,
+    val conversationType: String,
+    val authorUserId: String,
+    val type: String,
+    val encText: String?,
+    val mediaUrl: String?,
+    val durationMs: Long?,
+    val createdAt: Long,
+    val deleted: Boolean,
+    val reactionsJson: String
+)