feat: Implement secure database operations and caching for encrypted accounts

app/src/main/java/com/rosetta/messenger/database/EncryptedAccountEntity.kt

@@ -0,0 +1,38 @@
+package com.rosetta.messenger.database
+
+import androidx.room.ColumnInfo
+import androidx.room.Entity
+import androidx.room.Index
+import androidx.room.PrimaryKey
+
+/**
+ * Зашифрованный аккаунт в базе данных
+ * Соответствует структуре из React Native приложения
+ */
+@Entity(
+    tableName = "encrypted_accounts",
+    indices = [Index(value = ["public_key"], unique = true)]
+)
+data class EncryptedAccountEntity(
+    @PrimaryKey(autoGenerate = true)
+    @ColumnInfo(name = "id")
+    val id: Long = 0,
+
+    @ColumnInfo(name = "public_key")
+    val publicKey: String,
+
+    @ColumnInfo(name = "private_key_encrypted")
+    val privateKeyEncrypted: String,
+
+    @ColumnInfo(name = "seed_phrase_encrypted")
+    val seedPhraseEncrypted: String,
+
+    @ColumnInfo(name = "created_at")
+    val createdAt: String,
+
+    @ColumnInfo(name = "last_used")
+    val lastUsed: String? = null,
+
+    @ColumnInfo(name = "is_active")
+    val isActive: Boolean = true
+)