feat(#389): create SpeechModule with provider abstraction layer

Add SpeechModule with provider interfaces and service skeleton for
multi-tier TTS fallback (premium -> default -> fallback) and STT
transcription support. Includes 27 unit tests covering provider
selection, fallback logic, and availability checks.

- ISTTProvider interface with transcribe/isHealthy methods
- ITTSProvider interface with synthesize/listVoices/isHealthy methods
- Shared types: SpeechTier, TranscriptionResult, SynthesisResult, etc.
- SpeechService with graceful TTS fallback chain
- NestJS injection tokens (STT_PROVIDER, TTS_PROVIDERS)
- SpeechModule registered in AppModule
- ConfigModule integration via speechConfig registerAs factory

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

apps/api/src/app.module.ts

@@ -37,6 +37,7 @@ import { JobStepsModule } from "./job-steps/job-steps.module";
 import { CoordinatorIntegrationModule } from "./coordinator-integration/coordinator-integration.module";
 import { FederationModule } from "./federation/federation.module";
 import { CredentialsModule } from "./credentials/credentials.module";
+import { SpeechModule } from "./speech/speech.module";
 import { RlsContextInterceptor } from "./common/interceptors/rls-context.interceptor";
 
 @Module({
@@ -97,6 +98,7 @@ import { RlsContextInterceptor } from "./common/interceptors/rls-context.interce
     CoordinatorIntegrationModule,
     FederationModule,
     CredentialsModule,
+    SpeechModule,
   ],
   controllers: [AppController, CsrfController],
   providers: [