feat(#397): implement WebSocket streaming transcription gateway

Add SpeechGateway with Socket.IO namespace /speech for real-time
streaming transcription. Supports start-transcription, audio-chunk,
and stop-transcription events with session management, authentication,
and buffer size rate limiting. Includes 29 unit tests covering
authentication, session lifecycle, error handling, cleanup, and
client isolation.

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

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

@@ -11,15 +11,18 @@
  *
  * Imports:
  * - ConfigModule.forFeature(speechConfig) for speech configuration
+ * - AuthModule for WebSocket authentication
+ * - PrismaModule for workspace membership queries
  *
  * Providers:
  * - SpeechService: High-level speech operations with provider selection
+ * - SpeechGateway: WebSocket gateway for streaming transcription (Issue #397)
  * - TTS_PROVIDERS: Map<SpeechTier, ITTSProvider> populated by factory based on config
  *
  * Exports:
  * - SpeechService for use by other modules (e.g., controllers, brain)
  *
- * Issue #389, #390, #391
+ * Issue #389, #390, #391, #397
  */
 
 import { Module, type OnModuleInit, Logger } from "@nestjs/common";
@@ -32,15 +35,19 @@ import {
 } from "./speech.config";
 import { SpeechService } from "./speech.service";
 import { SpeechController } from "./speech.controller";
+import { SpeechGateway } from "./speech.gateway";
 import { STT_PROVIDER, TTS_PROVIDERS } from "./speech.constants";
 import { SpeachesSttProvider } from "./providers/speaches-stt.provider";
 import { createTTSProviders } from "./providers/tts-provider.factory";
+import { AuthModule } from "../auth/auth.module";
+import { PrismaModule } from "../prisma/prisma.module";
 
 @Module({
-  imports: [ConfigModule.forFeature(speechConfig)],
+  imports: [ConfigModule.forFeature(speechConfig), AuthModule, PrismaModule],
   controllers: [SpeechController],
   providers: [
     SpeechService,
+    SpeechGateway,
     // STT provider: conditionally register SpeachesSttProvider when STT is enabled
     ...(isSttEnabled()
       ? [