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/speech/speech.module.ts

@@ -0,0 +1,49 @@
+/**
+ * SpeechModule
+ *
+ * NestJS module for speech-to-text (STT) and text-to-speech (TTS) services.
+ * Provides a provider abstraction layer with graceful fallback for TTS tiers.
+ *
+ * Imports:
+ * - ConfigModule.forFeature(speechConfig) for speech configuration
+ *
+ * Providers:
+ * - SpeechService: High-level speech operations with provider selection
+ * - TTS_PROVIDERS: Empty Map<SpeechTier, ITTSProvider> (populated by provider modules)
+ *
+ * Exports:
+ * - SpeechService for use by other modules (e.g., controllers, brain)
+ *
+ * Issue #389
+ */
+
+import { Module, type OnModuleInit, Logger } from "@nestjs/common";
+import { ConfigModule } from "@nestjs/config";
+import { speechConfig, validateSpeechConfig } from "./speech.config";
+import { SpeechService } from "./speech.service";
+import { TTS_PROVIDERS } from "./speech.constants";
+import type { SpeechTier } from "./interfaces/speech-types";
+import type { ITTSProvider } from "./interfaces/tts-provider.interface";
+
+@Module({
+  imports: [ConfigModule.forFeature(speechConfig)],
+  providers: [
+    SpeechService,
+    // Default empty TTS providers map. Provider modules (Kokoro, Chatterbox, etc.)
+    // will register their providers in subsequent tasks.
+    {
+      provide: TTS_PROVIDERS,
+      useFactory: (): Map<SpeechTier, ITTSProvider> => new Map(),
+    },
+  ],
+  exports: [SpeechService],
+})
+export class SpeechModule implements OnModuleInit {
+  private readonly logger = new Logger(SpeechModule.name);
+
+  onModuleInit(): void {
+    // Validate configuration at startup (fail fast)
+    validateSpeechConfig();
+    this.logger.log("Speech module initialized");
+  }
+}