Don't jump progress bars back on stale Plex/Jellyfin positions

Media servers only learn the position when the player checks in (~10s),
so a poll can return an old number. A position up to 12s behind our
running estimate is now treated as stale; positions ahead, or further
behind (a rewind), are taken as-is. Phone-reported positions are exact
and always taken.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MzFywujuH9TiuXbr9vjoX8

apps/api/test/progress.test.ts

@@ -84,6 +84,33 @@ describe('realigning progress bars', () => {
     expect(later!.progressMs).toBeGreaterThanOrEqual(124_000);
   });
 
+  it('ignores a Plex position that is only a little behind (stale), but believes rewinds and jumps ahead', async () => {
+    const { user } = await createUser(app(), 'alice');
+    await applyPollResult(user.id, 'plex', playing(30_000));
+    const ago = (ms: number) =>
+      db.update(schema.nowPlaying).set({ updatedAt: new Date(Date.now() - ms) }).where(eq(schema.nowPlaying.userId, user.id));
+    const stored = async () => (await db.select().from(schema.nowPlaying).where(eq(schema.nowPlaying.userId, user.id)))[0]!.progressMs!;
+
+    await ago(10_000); // we'd expect ~40s now; Plex still says 33s
+    await applyPollResult(user.id, 'plex', playing(33_000));
+    expect(await stored()).toBeGreaterThanOrEqual(40_000);
+
+    await ago(0);
+    await applyPollResult(user.id, 'plex', playing(5_000)); // rewound to near the start
+    expect(await stored()).toBe(5_000);
+
+    await applyPollResult(user.id, 'plex', playing(90_000)); // skipped ahead
+    expect(await stored()).toBe(90_000);
+  });
+
+  it('takes phone-reported positions as they are', async () => {
+    const { user } = await createUser(app(), 'alice');
+    await applyPollResult(user.id, 'android_media_session', playing(30_000));
+    await applyPollResult(user.id, 'android_media_session', playing(25_000));
+    const [np] = await db.select().from(schema.nowPlaying).where(eq(schema.nowPlaying.userId, user.id));
+    expect(np!.progressMs).toBe(25_000);
+  });
+
   it("doesn't send anything extra for sources without a position", async () => {
     const { user } = await createUser(app(), 'alice');
     await applyPollResult(user.id, 'lastfm', playing(null));