feat(#391): add base TTS provider and factory classes

Add the BaseTTSProvider abstract class and TTS provider factory that were
part of the tiered TTS architecture but missed from the previous commit.

- BaseTTSProvider: abstract base with synthesize(), listVoices(), isHealthy()
- tts-provider.factory: creates Kokoro/Chatterbox/Piper providers from config
- 30 tests (22 base provider + 8 factory)

Refs #391

apps/api/src/speech/providers/tts-provider.factory.ts

@@ -0,0 +1,112 @@
+/**
+ * TTS Provider Factory
+ *
+ * Creates and registers TTS providers based on speech configuration.
+ * Reads enabled flags and URLs from config and instantiates the appropriate
+ * provider for each tier.
+ *
+ * Each tier maps to a specific TTS engine:
+ * - default: Kokoro-FastAPI (CPU, always available)
+ * - premium: Chatterbox (GPU, voice cloning)
+ * - fallback: Piper via OpenedAI Speech (ultra-lightweight CPU)
+ *
+ * Issue #391
+ */
+
+import { Logger } from "@nestjs/common";
+import { BaseTTSProvider } from "./base-tts.provider";
+import type { ITTSProvider } from "../interfaces/tts-provider.interface";
+import type { SpeechTier, AudioFormat } from "../interfaces/speech-types";
+import type { SpeechConfig } from "../speech.config";
+
+// ==========================================
+// Concrete provider classes
+// ==========================================
+
+/**
+ * Kokoro TTS provider (default tier).
+ * CPU-based, always available, Apache 2.0 license.
+ */
+class KokoroProvider extends BaseTTSProvider {
+  readonly name = "kokoro";
+  readonly tier: SpeechTier = "default";
+}
+
+/**
+ * Chatterbox TTS provider (premium tier).
+ * GPU required, voice cloning capable, MIT license.
+ */
+class ChatterboxProvider extends BaseTTSProvider {
+  readonly name = "chatterbox";
+  readonly tier: SpeechTier = "premium";
+
+  constructor(baseURL: string) {
+    super(baseURL, "default", "mp3");
+  }
+}
+
+/**
+ * Piper TTS provider via OpenedAI Speech (fallback tier).
+ * Ultra-lightweight CPU, GPL license.
+ */
+class PiperProvider extends BaseTTSProvider {
+  readonly name = "piper";
+  readonly tier: SpeechTier = "fallback";
+
+  constructor(baseURL: string) {
+    super(baseURL, "alloy", "mp3");
+  }
+}
+
+// ==========================================
+// Factory function
+// ==========================================
+
+const logger = new Logger("TTSProviderFactory");
+
+/**
+ * Create and register TTS providers based on the speech configuration.
+ *
+ * Only creates providers for tiers that are enabled in the config.
+ * Returns a Map keyed by SpeechTier for use with the TTS_PROVIDERS injection token.
+ *
+ * @param config - Speech configuration with TTS tier settings
+ * @returns Map of enabled TTS providers keyed by tier
+ */
+export function createTTSProviders(config: SpeechConfig): Map<SpeechTier, ITTSProvider> {
+  const providers = new Map<SpeechTier, ITTSProvider>();
+
+  // Default tier: Kokoro
+  if (config.tts.default.enabled) {
+    const provider = new KokoroProvider(
+      config.tts.default.url,
+      config.tts.default.voice,
+      config.tts.default.format as AudioFormat
+    );
+    providers.set("default", provider);
+    logger.log(`Registered default TTS provider: kokoro at ${config.tts.default.url}`);
+  }
+
+  // Premium tier: Chatterbox
+  if (config.tts.premium.enabled) {
+    const provider = new ChatterboxProvider(config.tts.premium.url);
+    providers.set("premium", provider);
+    logger.log(`Registered premium TTS provider: chatterbox at ${config.tts.premium.url}`);
+  }
+
+  // Fallback tier: Piper
+  if (config.tts.fallback.enabled) {
+    const provider = new PiperProvider(config.tts.fallback.url);
+    providers.set("fallback", provider);
+    logger.log(`Registered fallback TTS provider: piper at ${config.tts.fallback.url}`);
+  }
+
+  if (providers.size === 0) {
+    logger.warn("No TTS providers are enabled. TTS synthesis will not be available.");
+  } else {
+    const tierNames = Array.from(providers.keys()).join(", ");
+    logger.log(`TTS providers ready: ${tierNames} (${String(providers.size)} total)`);
+  }
+
+  return providers;
+}