feat(presenter): replace Web Speech API with Piper TTS for high-quality voice

- New API route /api/presenter/tts proxies to compliance-tts-service
- usePresenterMode now uses Audio element with Piper-generated MP3
- Client-side audio caching (text hash → blob URL) avoids re-synthesis
- Graceful fallback to word-count timer if TTS service unavailable
- Add TTS_SERVICE_URL env var to pitch-deck Docker config

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

pitch-deck/app/api/presenter/tts/route.ts

@@ -0,0 +1,46 @@
+import { NextRequest, NextResponse } from 'next/server'
+
+const TTS_SERVICE_URL = process.env.TTS_SERVICE_URL || 'http://compliance-tts-service:8095'
+
+export async function POST(request: NextRequest) {
+  try {
+    const body = await request.json()
+    const { text, language = 'de' } = body
+
+    if (!text || typeof text !== 'string') {
+      return NextResponse.json({ error: 'Text is required' }, { status: 400 })
+    }
+
+    const res = await fetch(`${TTS_SERVICE_URL}/synthesize-direct`, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ text, language }),
+      signal: AbortSignal.timeout(30000),
+    })
+
+    if (!res.ok) {
+      const errorText = await res.text()
+      console.error('TTS service error:', res.status, errorText)
+      return NextResponse.json(
+        { error: `TTS service error (${res.status})` },
+        { status: 502 }
+      )
+    }
+
+    const audioBuffer = await res.arrayBuffer()
+
+    return new NextResponse(audioBuffer, {
+      headers: {
+        'Content-Type': 'audio/mpeg',
+        'Cache-Control': 'public, max-age=86400', // Cache 24h — texts are static
+        'X-TTS-Cache': res.headers.get('X-TTS-Cache') || 'unknown',
+      },
+    })
+  } catch (error) {
+    console.error('TTS proxy error:', error)
+    return NextResponse.json(
+      { error: 'TTS service not reachable' },
+      { status: 503 }
+    )
+  }
+}