chore: merge Rust backend into monorepo

git-subtree-dir: server
git-subtree-mainline: fb6a84d75f
git-subtree-split: 8a88706ff9

server/.env.example

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+# The API starts without this variable, but data endpoints return HTTP 503.
+# DATABASE_URL=postgres://username:password@hostname:5432/teaching_feedback?sslmode=require
+
+HOST=127.0.0.1
+PORT=8080
+DATABASE_MAX_CONNECTIONS=5
+
+# A single development origin. Leave unset to disable CORS middleware.
+# CORS_ALLOWED_ORIGIN=https://servicewechat.com
+
+# Private server directory used to retain uploaded audio for transcription retry.
+AUDIO_STORAGE_DIR=./data/audio
+
+# Local FunASR is the default Chinese transcription provider.
+ASR_PROVIDER=local
+LOCAL_ASR_URL=http://127.0.0.1:10095
+ASR_REQUEST_TIMEOUT_SECONDS=1800
+ASR_WORKER_CONCURRENCY=1
+ASR_JOB_LEASE_SECONDS=3600
+
+# Optional Tencent Cloud fallback. Set ASR_PROVIDER=tencent and configure all values.
+# TENCENT_CLOUD_ASR_APP_ID=1234567890
+# TENCENT_CLOUD_ASR_SECRET_ID=your-secret-id
+# TENCENT_CLOUD_ASR_SECRET_KEY=your-secret-key
+TENCENT_CLOUD_ASR_ENGINE_TYPE=16k_zh
+
+# Optional OpenAI-compatible chat completions endpoint for polished feedback summaries.
+# Without it, the API uses a local extractive summary and remains functional.
+# FEEDBACK_SUMMARY_API_URL=https://api.example.com/v1/chat/completions
+# FEEDBACK_SUMMARY_API_KEY=your-api-key
+# FEEDBACK_SUMMARY_MODEL=your-model