fix(r5): address the fifth /code-review high round — 6 findings

- run_ffmpeg stops the process from ONE exit path (`except BaseException`), so
  an exception out of on_progress (a DB write) can no longer orphan a live
  ffmpeg after a 10s join on its pipes; _stop is now a no-op on a dead process
- both run_ffmpeg callbacks are throttled to ~1s (they were a DB round-trip per
  output line: ~10^5 queries on a long re-encode), mirroring the download hook
- volume arithmetic extracted to lib/playerVolume with unit tests; a corrupt
  level now falls back to the LAST GOOD one instead of full blast, and the
  sanitising happens where the stored value enters
- RSS is back to two except clauses (the shared tail already sits after the try)
- new tests: callback-exception cleanup, the two throttles; the fake Popen grew
  poll() and its wait count is now asserted

Mutation-checked: each new group goes red when its fix is reverted.

backend/app/worker.py

@@ -319,6 +319,46 @@ def _make_progress_hook(job_id: int, asset_id: int):
     return hook
 
 
+# ffmpeg's `-progress pipe:1` emits a block of ~12 lines every ~0.5s, and run_ffmpeg calls both of
+# its callbacks once per LINE. Each of these is a database round-trip (`_job_status` opens a session
+# and SELECTs; `_set_job` opens one and UPDATE+COMMITs), so an unthrottled pair costs ~10^5 queries
+# on a long re-encode. Both are therefore rate-limited to ~1s — the same 1s DB throttle the
+# download progress hook already applies (see `_make_progress_hook`).
+_FFMPEG_CALLBACK_INTERVAL_S = 1.0
+
+
+def _ffmpeg_progress_writer(job_id: int, phase: str):
+    """`on_progress` for run_ffmpeg: persist at most one row write per second, and only when the
+    whole-percent value actually changed (the bar shows integers)."""
+    state = {"at": 0.0, "pct": -1}
+
+    def write(pct: float) -> None:
+        whole = int(pct)
+        now = time.monotonic()
+        if whole == state["pct"] or now - state["at"] < _FFMPEG_CALLBACK_INTERVAL_S:
+            return
+        state["at"] = now
+        state["pct"] = whole
+        _set_job(job_id, progress=whole, phase=phase)
+
+    return write
+
+
+def _ffmpeg_cancel_poll(job_id: int):
+    """`should_cancel` for run_ffmpeg: ask the DB at most once a second and reuse the answer in
+    between. Cancellation is sticky, so once it says yes it never queries again."""
+    state = {"at": 0.0, "cancelled": False}
+
+    def cancelled() -> bool:
+        now = time.monotonic()
+        if not state["cancelled"] and now - state["at"] >= _FFMPEG_CALLBACK_INTERVAL_S:
+            state["at"] = now
+            state["cancelled"] = _job_status(job_id) not in ("running", None)
+        return state["cancelled"]
+
+    return cancelled
+
+
 def _pp_phase(pp: str) -> str:
     """Map a yt-dlp postprocessor class name to a user-facing phase label. ffmpeg post-steps
     aren't byte-progress operations (no %), so the UI shows the step name + an indeterminate
@@ -492,8 +532,8 @@ def _ensure_browser_playable(job_id: int, produced: Path, spec: dict, info: dict
     editmod.run_ffmpeg(
         editmod.build_browser_safe_cmd(produced, dest, reenc_v, reenc_a),
         total_s=total_s,
-        on_progress=lambda pct: _set_job(job_id, progress=int(pct), phase="processing"),
-        should_cancel=lambda: _job_status(job_id) not in ("running", None),
+        on_progress=_ffmpeg_progress_writer(job_id, "processing"),
+        should_cancel=_ffmpeg_cancel_poll(job_id),
     )
     produced.unlink(missing_ok=True)
     # Keep the stored asset metadata truthful about what's now on disk.
@@ -671,8 +711,8 @@ def _process_edit(job_id: int, asset_id: int) -> None:
         editmod.run_ffmpeg(
             cmd,
             total_s,
-            on_progress=lambda pct: _set_job(job_id, progress=int(pct), phase="editing"),
-            should_cancel=lambda: _job_status(job_id) not in ("running", None),
+            on_progress=_ffmpeg_progress_writer(job_id, "editing"),
+            should_cancel=_ffmpeg_cancel_poll(job_id),
         )
         if not dest_staging.exists():
             raise RuntimeError("ffmpeg produced no output file")