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.

frontend/src/lib/playerVolume.test.ts

@@ -0,0 +1,57 @@
+import { describe, expect, it } from "vitest";
+
+import { DEFAULT_VOLUME, normalizeVolume, stepVolume } from "./playerVolume";
+
+describe("normalizeVolume", () => {
+  it("keeps a valid level", () => {
+    expect(normalizeVolume(0)).toBe(0);
+    expect(normalizeVolume(37)).toBe(37);
+    expect(normalizeVolume(100)).toBe(100);
+  });
+
+  it("clamps out-of-range levels and rounds to whole percents", () => {
+    expect(normalizeVolume(-20)).toBe(0);
+    expect(normalizeVolume(150)).toBe(100);
+    expect(normalizeVolume(42.6)).toBe(43);
+  });
+
+  it.each([
+    ["a missing key", undefined],
+    ["an explicit null", null],
+    ["a stringified number", "80"],
+    ["NaN from a broken player", NaN],
+    ["a whole object", { volume: 50 }],
+  ])("falls back on %s", (_label, raw) => {
+    // The regression: a partial stored object made the level undefined, which Math.round turned
+    // into NaN — muting playback and persisting `null` for every later session.
+    expect(normalizeVolume(raw, 15)).toBe(15);
+  });
+
+  it("prefers the last known level over the default, so a quiet session stays quiet", () => {
+    expect(normalizeVolume(NaN, 15)).toBe(15);
+    expect(normalizeVolume(NaN, 15)).not.toBe(DEFAULT_VOLUME);
+  });
+
+  it("uses the default when the fallback is unusable too", () => {
+    expect(normalizeVolume(NaN, NaN)).toBe(DEFAULT_VOLUME);
+    expect(normalizeVolume(null, undefined as unknown as number)).toBe(DEFAULT_VOLUME);
+  });
+});
+
+describe("stepVolume", () => {
+  it("steps up and down from the current level", () => {
+    expect(stepVolume(60, 5, 100)).toBe(65);
+    expect(stepVolume(60, -5, 100)).toBe(55);
+  });
+
+  it("clamps at both ends instead of wrapping", () => {
+    expect(stepVolume(98, 5, 100)).toBe(100);
+    expect(stepVolume(3, -5, 100)).toBe(0);
+  });
+
+  it("steps from the fallback when the player answers garbage", () => {
+    // A spin must not lose notches to a stale/unusable reading — it continues from what we last set.
+    expect(stepVolume(undefined, -5, 90)).toBe(85);
+    expect(stepVolume(NaN, -5, 90)).toBe(85);
+  });
+});