`logger.error("[agent_chat] %s", e)` only printed the exception type
and message — the traceback was gone, which made debugging "An
unexpected error occurred" responses much harder than it needed to be.
Switch to `logger.exception(...)` so the stack lands in the log.

Also include `detail: "{ExcType}: {message}"` in the 500 response body
when DEBUG=True, so local consumers see what blew up without having to
tail server logs. Production (DEBUG=False) is unchanged.

Bumps to 0.7.2.