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/sync/runner.py

@@ -74,15 +74,16 @@ def run_rss_poll(db: Session, channels: list[Channel] | None = None) -> dict[str
             channel = futures[fut]
             try:
                 entries = fut.result()
-            except Exception as exc:
-                # A failed poll is NOT an empty feed: skip the apply so `last_rss_at` stays where
-                # it was. Stamping it here made a permanently-broken feed look freshly polled.
-                # An RssError is an EXPECTED outcome (404, timeout) → one warning line; anything
-                # else is a bug on our side and earns a traceback.
-                if isinstance(exc, RssError):
-                    log.warning("RSS poll skipped for channel %s: %s", channel.id, exc)
-                else:
-                    log.exception("RSS fetch failed for channel %s", channel.id)
+            # A failed poll is NOT an empty feed: skip the apply so `last_rss_at` stays where it
+            # was (stamping it made a permanently-broken feed look freshly polled). The shared
+            # counter/progress tail lives after the statement, so the two outcomes can stay two
+            # clauses: an RssError is EXPECTED (404, timeout) and gets one warning line; anything
+            # else is a bug on our side and earns a traceback.
+            except RssError as exc:
+                log.warning("RSS poll skipped for channel %s: %s", channel.id, exc)
+                failed += 1
+            except Exception:
+                log.exception("RSS fetch failed for channel %s", channel.id)
                 failed += 1
             else:
                 try: