chore(config): add backend environment variable example

.env.example

@@ -0,0 +1,24 @@
+# TarninaHub — backend runtime configuration example.
+# Copy to `.env` (not tracked) or provide as environment variables.
+# Never commit real secrets.
+
+ASPNETCORE_ENVIRONMENT=Production
+ASPNETCORE_URLS=http://+:8080
+
+# SurrealDB — embedded RocksDB by default.
+SurrealDb__Namespace=hubmi
+SurrealDb__Database=hubmi
+SurrealDb__DataDirectory=data
+
+# SurrealDB — remote mode (optional; leave DataDirectory unused when set).
+# SurrealDb__RemoteUrl=ws://127.0.0.1:8010/rpc
+# SurrealDb__Username=
+# SurrealDb__Password=
+
+# LLM (F-8, matchmaking) — runtime secret only.
+# LLM__ApiKey=
+
+# Observability (optional).
+# Logging__LogLevel__Default=Information
+# OTEL_EXPORTER_OTLP_ENDPOINT=
+# OTEL_SERVICE_NAME=tarninahub