The 600s wall-clock cap in research_handler.start_research was too short
for local / edge LLMs to finish a deep-research synthesis — long
extraction passes plus a slow final report routinely blew past 10
minutes and the run was killed with partial results.
Introduce research_run_timeout_seconds (default 1800s = 30 min) in
DEFAULT_SETTINGS and resolve it at start_research entry when the caller
hasn't pinned hard_timeout. Bound the resolved value at [60, 86400] so a
misconfigured settings.json can't either disable the safety net or
explode into a multi-day hang. Existing call sites in research_routes.py
and chat_routes.py keep working unchanged — they don't pass hard_timeout
and now pick up the new default.
Closes#595.
