fix(downloads): heartbeat only when no write touched the row this interval (E3 review R2)

Round-2 review of the R1 fixes found that the two-independent-limiter progress
writer issued a redundant heartbeat every interval during an ACTIVE download: a
percent change writes progress (which already refreshes updated_at, the stall
watchdog's liveness source), then the sub-integer calls that follow fire the
separate beat budget — ~2 DB writes/interval instead of the throttle's intended 1.

Replace the second limiter with a shared `last_touch` clock (last DB write OR
beat). Progress writes keep their own `allow_write` budget (so a beat still can't
delay a real write — the R1 concern), but the heartbeat now fires only when
nothing touched the row for a full interval. Active download → ~1 write/interval;
a genuinely stuck re-encode → 1 heartbeat/interval. New regression test:
an unchanged-percent call in the same interval as a write does NOT heartbeat.

ruff clean, full DB-lane 365 pass.

backend/tests/test_ffmpeg_callbacks.py

@@ -60,6 +60,21 @@ class TestProgressWriter:
         assert [w["progress"] for w in writes] == [10]  # progress never rewritten
         assert touches == [7, 7, 7]  # …but each interval heartbeats the watchdog
 
+    def test_no_redundant_heartbeat_when_a_progress_write_just_happened(self, monkeypatch):
+        """A progress write already refreshes updated_at, so an unchanged-percent call in the SAME
+        interval must NOT also heartbeat — that would be a redundant DB write (~2x during an active
+        download). The heartbeat only fires once no write/beat touched the row for an interval."""
+        writes, touches = [], []
+        clock = {"t": 1000.0}
+        monkeypatch.setattr(pipeline, "_set_job", lambda job_id, **f: writes.append(f))
+        monkeypatch.setattr(pipeline, "_touch_job", lambda job_id: touches.append(job_id))
+        write = pipeline._ffmpeg_progress_writer(7, "processing", now=lambda: clock["t"])
+        write(6.0)  # progress write at t=1000 → refreshes updated_at
+        clock["t"] += 0.3
+        write(6.4)  # same percent, SAME interval → the write already touched the row, so no heartbeat
+        assert [w["progress"] for w in writes] == [6]
+        assert touches == []
+
 
 class TestCancelPoll:
     def test_asks_once_then_reuses_the_answer(self, monkeypatch):