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/interfaces/index.ts

@@ -0,0 +1,18 @@
+/**
+ * Speech interfaces barrel export.
+ *
+ * Issue #389
+ */
+
+export type { ISTTProvider } from "./stt-provider.interface";
+export type { ITTSProvider } from "./tts-provider.interface";
+export type {
+  SpeechTier,
+  AudioFormat,
+  TranscribeOptions,
+  TranscriptionResult,
+  TranscriptionSegment,
+  SynthesizeOptions,
+  SynthesisResult,
+  VoiceInfo,
+} from "./speech-types";