fix vibe engine audit findings: pg.Pool, plan replan, dead exclusions, legacy engine removal

Backend:
- app.ts: switch shared pg.Client to pg.Pool with per-transaction clients (#205)
- v2.routes.ts: replace plan instead of appending on replan, fixing self-duplication (#206)
- session-director: populate recentExclusions, per-candidate ranking, batch repetition checks (#209/#211/#213/#215 + minor)
- db.service.ts: claim-fusion watermark, legacy recommendation_batch engine removed (#216/#219/#232)
- app.ts: drop test enqueue-job endpoint (#234)

Frontend:
- AudioEngine/Vibe/usePlaybackStore: dedupe completed feedback, gate feedback to vibe sessions, End Vibe stops playback, Keep toast, shuffle played-set (#207/#236/#237/#238/#239/#240)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/components/AudioEngine.tsx

@@ -1,9 +1,18 @@
 import { useEffect, useRef } from 'react';
 import { usePlaybackStore } from '../store/usePlaybackStore';
+import { useVibeStore } from '../store/useVibeStore';
 import { trackService } from '../services/trackService';
 import { vibeService } from '../services/vibeService';
 import type { Track } from '../types';
 
+// Track ids whose next natural feedback transition should be skipped because
+// the caller (e.g. Vibe.tsx's dislike button) already recorded feedback for
+// them explicitly. Consumed once, then cleared.
+const suppressedFeedbackIds = new Set<string>();
+export function suppressAutoFeedback(trackId: string): void {
+  suppressedFeedbackIds.add(trackId);
+}
+
 // Threshold (seconds) above which a store position change is treated as a user
 // scrub and applied to the audio element. Keeps the timeupdate -> setPosition ->
 // effect loop from fighting itself.
@@ -39,9 +48,6 @@ export const AudioEngine = () => {
   const endedNaturallyRef = useRef(false);
   // Track whether the current track has crossed the completion threshold.
   const crossedThresholdRef = useRef(false);
-  // Track whether we've already recorded a completed play for the current track
-  // (to avoid double-recording when both threshold crossed AND ended fires).
-  const recordedCompletedRef = useRef(false);
 
   // --- DOM -> store: media events -----------------------------------------
   useEffect(() => {
@@ -73,16 +79,10 @@ export const AudioEngine = () => {
       if (store().isPlaying) store().pause();
     };
     const onEnded = () => {
-      const trackId = loadedIdRef.current;
-      if (trackId && !recordedCompletedRef.current) {
-        endedNaturallyRef.current = true;
-        recordedCompletedRef.current = true;
-        try {
-          void vibeService.feedback(trackId, 'completed').catch(() => {});
-        } catch {
-          /* best-effort */
-        }
-      }
+      // Just flag it — applyTrack (below) is the single place that sends
+      // feedback, on the resulting track-change, so completion is recorded
+      // exactly once per track.
+      endedNaturallyRef.current = true;
       store().next();
     };
 
@@ -114,21 +114,23 @@ export const AudioEngine = () => {
       // If it crossed the threshold OR ended naturally, record as completed.
       const prevId = loadedIdRef.current;
       const completed = endedNaturallyRef.current || crossedThresholdRef.current;
-      if (prevId) {
-        try {
-          if (completed) {
-            recordedCompletedRef.current = true;
-            void vibeService.feedback(prevId, 'completed').catch(() => {});
-          } else {
-            void vibeService.feedback(prevId, 'skipped').catch(() => {});
+      // Only vibe sessions want this feedback — plain library browsing
+      // shouldn't write skip/completed evidence for tracks merely sampled.
+      const inVibeSession = !!useVibeStore.getState().activeSessionId;
+      if (prevId && inVibeSession) {
+        if (suppressedFeedbackIds.delete(prevId)) {
+          // Caller already recorded explicit feedback (e.g. dislike) for
+          // this track — don't also record the implicit transition.
+        } else {
+          try {
+            void vibeService.feedback(prevId, completed ? 'completed' : 'skipped').catch(() => {});
+          } catch {
+            /* best-effort */
           }
-        } catch {
-          /* best-effort */
         }
       }
       endedNaturallyRef.current = false;
       crossedThresholdRef.current = false;
-      recordedCompletedRef.current = false;
       loadedIdRef.current = id;
 
       if (!id) {