feat(voice): self-hosted speech recognition via Whisper (STT_*)

- STT_URL switches voice input to an OpenAI-compatible Whisper server
  (e.g. speaches); without it the browser's recognition stays. Configured
  by environment only, with limits for upload size, parallel jobs, hourly
  budget per user and timeout. The demo always uses the browser.
- POST /voice/transcribe forwards recordings and returns the text; nothing
  is stored. On startup MindSift waits for the server and asks it to
  download the model if missing.
- Admin card shows the active mode and server state, with a test recording.
- docker-compose: optional "whisper" service (speaches 0.8.3 CPU, profile
  "whisper", models in ./whisper).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/lib/api.ts

@@ -35,6 +35,7 @@ import {
   TagSummary,
   User,
   UserInvite,
+  VoiceAdminStatus,
   WSMessage,
 } from './types';
 import { extractHashtags } from './hashtags';
@@ -1047,6 +1048,26 @@ export const api = {
     await request(`/api/v1/admin/invites/${id}`, { method: 'DELETE' });
   },
 
+  async adminVoiceStatus(): Promise<VoiceAdminStatus> {
+    return request<VoiceAdminStatus>('/api/v1/admin/voice');
+  },
+
+  // Sends a recording to the own Whisper server (voice mode "server") and returns the text
+  async transcribe(audio: Blob): Promise<string> {
+    const token = getToken();
+    const ext = audio.type.includes('mp4') ? 'm4a' : audio.type.includes('ogg') ? 'ogg' : 'webm';
+    const form = new FormData();
+    form.append('file', audio, `aufnahme.${ext}`);
+    const res = await fetch('/api/v1/voice/transcribe', {
+      method: 'POST',
+      headers: token ? { Authorization: `Bearer ${token}` } : {},
+      body: form,
+    });
+    const data = await res.json().catch(() => ({}));
+    if (!res.ok) throw new Error(data.error || `Spracherkennung fehlgeschlagen (HTTP ${res.status})`);
+    return data.text || '';
+  },
+
   async adminGetSettings(): Promise<SystemSettings> {
     return request<SystemSettings>('/api/v1/admin/settings');
   },