feat(logging): integrate slog logging with env control and request/sync tracing

- Configure structured slog logger in cmd/server/main.go via LOG_LEVEL and DEBUG env vars
- Add HTTP API request logging middleware in internal/api/ with status and duration_ms
- Add WebSocket join/leave, event reception, and debounced SQLite flush tracing in docroom
- Document LOG_LEVEL and DEBUG settings in .env.example, compose.yaml, and docker-compose.yml

.env.example

@@ -5,7 +5,10 @@ DATABASE_URL=sqlite:///data/mindsift.db
 JWT_SECRET=change_this_to_a_secure_random_key_min_32_chars
 BASE_URL=http://localhost:8080
 ALLOW_REGISTRATION=false
+# Logging level: debug, info, warn, error (Default: info)
 LOG_LEVEL=info
+# LOG_LEVEL=debug
+# DEBUG=true
 
 # Optional SMTP Server Configuration for Email Alarms (can also be set in Admin Settings UI)
 SMTP_HOST=