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/text-validation.pipe.ts

@@ -0,0 +1,65 @@
+/**
+ * TextValidationPipe
+ *
+ * NestJS PipeTransform that validates text input for TTS synthesis.
+ * Checks that text is non-empty and within the configurable maximum length.
+ *
+ * Usage:
+ * ```typescript
+ * @Post('synthesize')
+ * async synthesize(
+ *   @Body('text', new TextValidationPipe()) text: string,
+ * ) { ... }
+ * ```
+ *
+ * Issue #398
+ */
+
+import { BadRequestException } from "@nestjs/common";
+import type { PipeTransform } from "@nestjs/common";
+
+/**
+ * Default maximum text length for TTS input (4096 characters).
+ */
+const DEFAULT_MAX_TEXT_LENGTH = 4096;
+
+/**
+ * Options for customizing TextValidationPipe behavior.
+ */
+export interface TextValidationPipeOptions {
+  /** Maximum text length in characters. Defaults to 4096. */
+  maxTextLength?: number;
+}
+
+export class TextValidationPipe implements PipeTransform<string | null | undefined> {
+  private readonly maxTextLength: number;
+
+  constructor(options?: TextValidationPipeOptions) {
+    this.maxTextLength = options?.maxTextLength ?? DEFAULT_MAX_TEXT_LENGTH;
+  }
+
+  /**
+   * Validate the text input for TTS synthesis.
+   *
+   * @param text - The text to validate
+   * @returns The validated text, unchanged
+   * @throws {BadRequestException} If text is empty, whitespace-only, or exceeds the max length
+   */
+  transform(text: string | null | undefined): string {
+    if (text === null || text === undefined) {
+      throw new BadRequestException("Text cannot be empty");
+    }
+
+    if (text.trim().length === 0) {
+      throw new BadRequestException("Text cannot be empty");
+    }
+
+    if (text.length > this.maxTextLength) {
+      throw new BadRequestException(
+        `Text length ${String(text.length)} exceeds maximum allowed length of ${String(this.maxTextLength)} characters`
+      );
+    }
+
+    return text;
+  }
+}