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.constants.ts

@@ -0,0 +1,19 @@
+/**
+ * Speech Module Constants
+ *
+ * NestJS injection tokens for speech providers.
+ *
+ * Issue #389
+ */
+
+/**
+ * Injection token for the STT (speech-to-text) provider.
+ * Providers implementing ISTTProvider register under this token.
+ */
+export const STT_PROVIDER = Symbol("STT_PROVIDER");
+
+/**
+ * Injection token for TTS (text-to-speech) providers map.
+ * Registered as Map<SpeechTier, ITTSProvider>.
+ */
+export const TTS_PROVIDERS = Symbol("TTS_PROVIDERS");