Community scripts (build.func) install packages like Java/OpenSearch and
can take minutes, well past the nginx/proxy HTTP timeout. execute_script
now returns immediately with status "running" and finishes the job on a
background thread with its own DB session; the frontend polls
/api/scripts/executions/{id} every 3s while running.
Also set TERM=xterm for script execution over SSH — community scripts
call `clear` and print colored output, which fails with "TERM
environment variable not set" when run without a PTY.
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