feat(web): backflow play-event server-side classification

Web client now always POSTs play_ended with the actual position
played to, matching the Android fix at d82e744d (Scribe 519). The
server's skip rule (skip_max_completion_ratio +
skip_max_duration_played_ms, default 50%/30s) classifies was_skipped
from the duration_played_ms; the dispatcher no longer makes the
call itself.

Three transitions all collapse to play_ended:
- Track change (was: reachedEnd ? play_ended : play_skipped)
- Natural end via paused-at-duration (was already play_ended)
- pagehide via sendBeacon (was: play_skipped)

Drops the openReachedEnd field, COMPLETION_TOLERANCE_MS constant,
and the position-vs-duration threshold check that backed the prior
classification. play_skipped wire endpoint stays in place for a
future explicit-dislike affordance.

Two test cases updated:
- "user-initiated next mid-track" now asserts play_ended with
  duration_played_ms = the partial position, not play_skipped.
- "pagehide fires sendBeacon" now parses the beacon payload and
  asserts type=play_ended + duration_played_ms = last position.

Flutter NOT touched — deprecated per M8.

web/src/lib/player/events.svelte.ts

@@ -6,36 +6,34 @@ import type { PlayStartedResponse, EventOkResponse } from '$lib/api/types';
 // useEventsDispatcher installs $effect-based watchers that emit play events
 // to /api/events. Call once from +layout.svelte's mount.
 //
-// The dispatcher is a small state machine over (current track id, state).
-// Per spec data-flow:
+// The dispatcher is a small state machine over (current track id, state):
 //   - First transition into 'playing' for a new track → POST play_started.
-//   - Track id changes while a play is open → close the prior row:
-//     play_ended if it reached ~its duration, else play_skipped (#426).
-//     Without this every auto-advanced in-queue track was reported as a
-//     skip, force-flagging was_skipped=true server-side and poisoning
-//     the recommendation skip-ratio. Mirrors the Flutter reporter.
+//   - Track id changes while a play is open → close the prior row with
+//     play_ended carrying the actual position played to. Server applies
+//     its skip rule (skip_max_completion_ratio + skip_max_duration_played_ms,
+//     default 50%/30s) and decides was_skipped from that.
 //   - Natural completion (state moves from 'playing' to 'paused' AND
-//     position >= duration > 0) → POST play_ended on the open row
-//     (covers the last track / explicit pause-at-end).
-//   - pagehide with a live row → navigator.sendBeacon a play_skipped.
-
-// Within this much of the known duration counts as "finished", not a
-// skip. Matches the Flutter PlayEventsReporter tolerance.
-const COMPLETION_TOLERANCE_MS = 3000;
+//     position >= duration > 0) → POST play_ended on the open row.
+//   - pagehide with a live row → navigator.sendBeacon a play_ended with
+//     the last position. The server's rule still classifies skip-vs-not.
+//
+// Earlier shape made the skip-vs-ended call client-side via a "within 3s
+// of duration" rule, which left the server's threshold dead code and
+// treated every "next" tap as a skip even after most of the track played.
+// Backflow of the Android fix (commit d82e744d, 2026-06-01); see Scribe
+// task 519. Flutter client is deprecated and not updated.
 
 export function useEventsDispatcher(): void {
   let openPlayEventId: string | null = null;
   let openTrackId: string | null = null;
   // lastPositionMs tracks the *current* track's position (used by the
-  // pagehide beacon). openLastPositionMs / openReachedEnd track the
-  // *open row's* track specifically and are updated ONLY while that
-  // track is current — so a track change (which synchronously resets
-  // the store's position to 0 and duration to 0) can't clobber them
-  // before the close branch reads them. This is the #426 race fix.
+  // pagehide beacon). openLastPositionMs tracks the *open row's* track
+  // specifically and is updated ONLY while that track is current — so a
+  // track change (which synchronously resets the store's position to 0
+  // and duration to 0) can't clobber it before the close branch reads
+  // it. This is the #426 race fix.
   let lastPositionMs = 0;
   let openLastPositionMs = 0;
-  let openDurationMs = 0;
-  let openReachedEnd = false;
   let prevTrackId: string | null = null;
   let prevState: string | null = null;
   const clientId = getOrCreateClientId();
@@ -45,12 +43,6 @@ export function useEventsDispatcher(): void {
     lastPositionMs = posMs;
     if (openTrackId && player.current?.id === openTrackId) {
       openLastPositionMs = posMs;
-      if (player.duration > 0) {
-        openDurationMs = Math.round(player.duration * 1000);
-        if (posMs >= openDurationMs - COMPLETION_TOLERANCE_MS) {
-          openReachedEnd = true;
-        }
-      }
     }
   });
 
@@ -60,25 +52,20 @@ export function useEventsDispatcher(): void {
     const s = player.state;
     const tid = t?.id ?? null;
 
-    // Track changed: close the prior open row. If that track reached
-    // ~its duration it finished naturally (auto-advance) → play_ended
-    // with its full duration; otherwise the user moved on early →
-    // play_skipped at the last position it actually reached (#426).
+    // Track changed: close the prior open row. Always send play_ended
+    // with the actual position played; the server's skip rule decides
+    // whether to flag was_skipped.
     if (tid !== prevTrackId && openPlayEventId) {
       const id = openPlayEventId;
-      const finished = openReachedEnd;
-      const endedMs = openDurationMs > 0 ? openDurationMs : openLastPositionMs;
-      const skipMs = openLastPositionMs;
+      const dur = openLastPositionMs;
       openPlayEventId = null;
       openTrackId = null;
-      openDurationMs = 0;
       openLastPositionMs = 0;
-      openReachedEnd = false;
-      api.post<EventOkResponse>('/api/events',
-        finished
-          ? { type: 'play_ended', play_event_id: id, duration_played_ms: endedMs }
-          : { type: 'play_skipped', play_event_id: id, position_ms: skipMs }
-      ).catch(() => {});
+      api.post<EventOkResponse>('/api/events', {
+        type: 'play_ended',
+        play_event_id: id,
+        duration_played_ms: dur
+      }).catch(() => {});
     }
 
     // Entered playing for a new track (no open row yet for this track).
@@ -98,9 +85,7 @@ export function useEventsDispatcher(): void {
       const dur = lastPositionMs;
       openPlayEventId = null;
       openTrackId = null;
-      openDurationMs = 0;
       openLastPositionMs = 0;
-      openReachedEnd = false;
       api.post<EventOkResponse>('/api/events', {
         type: 'play_ended',
         play_event_id: id,
@@ -112,14 +97,16 @@ export function useEventsDispatcher(): void {
     prevState = s;
   });
 
-  // Page close: sendBeacon a play_skipped if an open play exists.
+  // Page close: sendBeacon a play_ended if an open play exists. The
+  // server's skip rule decides classification from the duration we
+  // actually played.
   if (typeof window !== 'undefined') {
     const onPageHide = () => {
       if (!openPlayEventId) return;
       const payload = JSON.stringify({
-        type: 'play_skipped',
+        type: 'play_ended',
         play_event_id: openPlayEventId,
-        position_ms: lastPositionMs
+        duration_played_ms: lastPositionMs
       });
       navigator.sendBeacon('/api/events', new Blob([payload], { type: 'application/json' }));
     };