docs: add .env.example with Mistral AI configuration placeholders

Added .env.example file with all AI service configuration placeholders.
Users should copy this to .env and fill in their actual API keys.
The .env file is already in .gitignore and will not be committed.

This allows users to configure Mistral, Vosk, and Coqui services without
committing sensitive API keys to git.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

.env.example

@@ -0,0 +1,44 @@
+# DeutschLernen Environment Configuration
+# Copy this file to .env and fill in your actual values
+# This file is in .gitignore and will NOT be committed to git
+
+# ============================================
+# JWT Configuration
+# ============================================
+JWT_KEY=your-super-secret-key-at-least-32-characters-long
+JWT_ISSUER=DeutschLernen
+JWT_AUDIENCE=DeutschLernen
+JWT_EXPIREHOURS=24
+
+# ============================================
+# Mistral AI Configuration
+# Use the API key from appsettings.Development.json
+# ============================================
+MISTRAL_APIKEY=YOUR_MISTRAL_API_KEY_HERE
+MISTRAL_BASEURL=https://api.mistral.ai/v1/
+MISTRAL_DEFAULTMODEL=mistral-medium
+MISTRAL_TIMEOUTSECONDS=30
+MISTRAL_MAXRETRIES=3
+MISTRAL_RATELIMITPERMINUTE=10
+MISTRAL_ENABLECACHING=true
+MISTRAL_CACHETTLMINUTES=60
+MISTRAL_CIRCUITBREAKERFAILURETHRESHOLD=5
+MISTRAL_CIRCUITBREAKERRESETMINUTES=1
+
+# ============================================
+# Vosk Speech Recognition Configuration
+# ============================================
+VOSK_PYTHONPATH=python3
+VOSK_MODELPATH=./models/vosk-model-de-0.22
+VOSK_SAMPLERATE=16000
+VOSK_TIMEOUTSECONDS=30
+VOSK_BEAMWIDTH=20
+
+# ============================================
+# Coqui TTS Configuration
+# ============================================
+COQUI_PYTHONPATH=python3
+COQUI_MODELNAME=tts_models/de/deu/fairseq/vits
+COQUI_AUDIOSTORAGEPATH=./tmp/tts-audio
+COQUI_MAXTEXTLENGTH=5000
+COQUI_TIMEOUTSECONDS=60