fix(lobe-chat): repoint the System Agent off its gpt-5-mini default onto fleet models

The gpt-5-mini calls were Lobe's System Agent -- a background model,
separate from the chat model, used for auto-naming conversations, history
summarization, translation, query rewrite, thread naming, and assistant
metadata. Its default is openai/gpt-5-mini, which our OpenAI provider (the
gateway) forwards verbatim; the scoped key blocks it, so every background
task 403'd and the log filled with 'Tried to access gpt-5-mini' while
auto-naming silently failed.

Set SYSTEM_AGENT to route all six documented keys (topic, translation,
agentMeta, queryRewrite, historyCompress, thread) at fleet models --
summarizer for the naming/summarize tasks (same seat as gen at temp 0),
gen where quality matters. Any key left unset falls back to the gpt-5-mini
default, so all six are explicit.

Notably this one IS env-configurable (SYSTEM_AGENT), unlike the per-model
output-token cap which is UI-only -- a mixed result on the
manageable-by-agent axis.

stacks/lobe-chat/.env.example

@@ -8,3 +8,5 @@ OPENAI_PROXY_URL=http://10.250.50.70:4000/v1
 OPENAI_API_KEY=
 ACCESS_CODE=
 KEY_VAULTS_SECRET=
+# System-agent background model routing (avoids the gpt-5-mini default 403ing on the scoped key)
+SYSTEM_AGENT=topic=openai/summarizer,translation=openai/gen,agentMeta=openai/gen,queryRewrite=openai/summarizer,historyCompress=openai/summarizer,thread=openai/summarizer