feat(#392): create /api/speech/transcribe REST endpoint

Add SpeechController with POST /api/speech/transcribe for audio
transcription and GET /api/speech/health for provider status.
Uses AudioValidationPipe for file upload validation and returns
results in standard { data: T } envelope.

Includes 10 unit tests covering transcribe with options, error
propagation, and all health status combinations.

Fixes #392

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/api/src/speech/speech.module.ts

@@ -31,12 +31,14 @@ import {
   type SpeechConfig,
 } from "./speech.config";
 import { SpeechService } from "./speech.service";
+import { SpeechController } from "./speech.controller";
 import { STT_PROVIDER, TTS_PROVIDERS } from "./speech.constants";
 import { SpeachesSttProvider } from "./providers/speaches-stt.provider";
 import { createTTSProviders } from "./providers/tts-provider.factory";
 
 @Module({
   imports: [ConfigModule.forFeature(speechConfig)],
+  controllers: [SpeechController],
   providers: [
     SpeechService,
     // STT provider: conditionally register SpeachesSttProvider when STT is enabled