feat(#398): add audio/text validation pipes and speech DTOs

Create AudioValidationPipe for MIME type and file size validation,
TextValidationPipe for TTS text input validation, and DTOs for
transcribe/synthesize endpoints. Includes 36 unit tests.

Fixes #398

apps/api/src/speech/dto/index.ts

@@ -0,0 +1,8 @@
+/**
+ * Speech DTOs barrel export
+ *
+ * Issue #398
+ */
+
+export { TranscribeDto } from "./transcribe.dto";
+export { SynthesizeDto } from "./synthesize.dto";