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/pipes/index.ts

@@ -0,0 +1,10 @@
+/**
+ * Speech Pipes barrel export
+ *
+ * Issue #398
+ */
+
+export { AudioValidationPipe } from "./audio-validation.pipe";
+export type { AudioValidationPipeOptions } from "./audio-validation.pipe";
+export { TextValidationPipe } from "./text-validation.pipe";
+export type { TextValidationPipeOptions } from "./text-validation.pipe";