fix(research): stop rescanning the research dir on every status poll (#3637)

get_status() called get_avg_duration() unconditionally, and that helper globs
and JSON-parses every file under the research data dir. The SSE status stream
polls get_status() roughly once a second, so with a few saved reports each poll
re-read and re-parsed all of them, including for sessions that are not active
(the disk branch never even used the value).

Compute avg_duration only for active sessions and memoize it on the task entry,
so a long stream computes it once instead of on every poll. Behaviour is
unchanged: active streams still report avg_duration.

Adds tests/test_research_status_avg_duration.py: an inactive session does no
avg scan, and an active session computes it once across many polls.

tests/test_research_status_avg_duration.py

@@ -0,0 +1,41 @@
+"""get_status must not rescan the whole research dir on every SSE poll.
+
+get_avg_duration() globs and JSON-parses every file under the research data dir.
+get_status() called it unconditionally on each poll, including for sessions that
+are not active (the common case while a client polls a finished report). It is
+now computed only for active sessions and memoized on the entry.
+"""
+from src.research_handler import ResearchHandler
+
+
+def _handler():
+    h = ResearchHandler.__new__(ResearchHandler)
+    h._active_tasks = {}
+    return h
+
+
+def test_inactive_session_does_not_compute_avg(monkeypatch):
+    h = _handler()
+    calls = []
+    monkeypatch.setattr(h, "get_avg_duration", lambda: (calls.append(1), 5.0)[1])
+    # Unknown session, no disk file -> None, and no expensive avg scan.
+    assert h.get_status("missing-session") is None
+    assert calls == []
+
+
+def test_active_session_memoizes_avg(monkeypatch):
+    h = _handler()
+    h._active_tasks["s1"] = {
+        "status": "running", "progress": {}, "query": "q", "started_at": 0,
+    }
+    calls = []
+    monkeypatch.setattr(h, "get_avg_duration", lambda: (calls.append(1), 12.0)[1])
+
+    r1 = h.get_status("s1")
+    r2 = h.get_status("s1")
+    r3 = h.get_status("s1")
+
+    assert r1["avg_duration"] == 12.0
+    assert r2["avg_duration"] == 12.0 and r3["avg_duration"] == 12.0
+    # Computed once across many polls, not once per poll.
+    assert len(calls) == 1