refactor: rename canto-backend → backend, canto-frontend → frontend

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

backend/.env.example

@@ -0,0 +1,26 @@
+SECRET_KEY=your-secret-key-change-this-in-production
+ALGORITHM=HS256
+ACCESS_TOKEN_EXPIRE_MINUTES=30
+DATABASE_URL=sqlite:///./qwen_tts.db
+CACHE_DIR=./voice_cache
+OUTPUT_DIR=./outputs
+MODEL_DEVICE=cuda:0
+MODEL_BASE_PATH=./Qwen
+MAX_CACHE_ENTRIES=100
+CACHE_TTL_DAYS=7
+HOST=0.0.0.0
+PORT=8000
+WORKERS=1
+LOG_LEVEL=info
+LOG_FILE=./app.log
+RATE_LIMIT_PER_MINUTE=50
+RATE_LIMIT_PER_HOUR=1000
+MAX_QUEUE_SIZE=100
+BATCH_SIZE=4
+BATCH_WAIT_TIME=0.5
+MAX_TEXT_LENGTH=1000
+MAX_AUDIO_SIZE_MB=10
+
+ALIYUN_REGION=beijing
+
+DEFAULT_BACKEND=local