Add partner SMS gateway via Gama SendQuick and document it.

Expose POST /public/sms/send with API key + domain allowlist for external backends like Balout, wire Meshkee OTP/message sends to Gama, and publish Partner SMS docs on /docs/website.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/public-sms/dto/send-public-sms.dto.ts

@@ -0,0 +1,19 @@
+import { IsNotEmpty, IsString, MaxLength, MinLength } from 'class-validator';
+
+export class SendPublicSmsDto {
+  @IsString()
+  @IsNotEmpty()
+  @MaxLength(253)
+  domain!: string;
+
+  @IsString()
+  @IsNotEmpty()
+  @MaxLength(32)
+  to!: string;
+
+  @IsString()
+  @IsNotEmpty()
+  @MinLength(1)
+  @MaxLength(700)
+  message!: string;
+}