feat: Implement end-to-end encryption for channels

- Added EncryptionSalt and WrappedRoomKey properties to Channel model.
- Introduced RoomCrypto class for client-side encryption and decryption.
- Updated ChannelService to handle encrypted channels, including creation and rekeying.
- Modified ChannelsController to expose crypto metadata and rekey functionality.
- Enhanced IrcCommandHandler to block joining encrypted channels over IRC.
- Updated database schema with migration for new encryption fields.
- Refactored file validation and image processing services to accommodate encrypted channels.
- Added unit tests for RoomCrypto functionality and updated existing tests for channel services.

src/EchoHub.Server/Data/Migrations/20260716012917_AddChannelEncryptionEnvelope.cs

@@ -0,0 +1,40 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace EchoHub.Server.Data.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddChannelEncryptionEnvelope : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<string>(
+                name: "EncryptionSalt",
+                table: "Channels",
+                type: "TEXT",
+                maxLength: 64,
+                nullable: true);
+
+            migrationBuilder.AddColumn<string>(
+                name: "WrappedRoomKey",
+                table: "Channels",
+                type: "TEXT",
+                maxLength: 200,
+                nullable: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "EncryptionSalt",
+                table: "Channels");
+
+            migrationBuilder.DropColumn(
+                name: "WrappedRoomKey",
+                table: "Channels");
+        }
+    }
+}