fix: give the worker a pg Pool and real transactions

The worker ran every job through a single pg Client while BullMQ was
configured with concurrency: 10. A Client is one connection with one
protocol stream and no queueing: ten concurrent jobs interleave on it, and
any BEGIN/COMMIT is shared by all of them, so an unrelated job's failure can
roll back another's work and a rollback can discard a third's committed
intent.

Switched to a Pool, added a small withTransaction(pool, fn) helper that
takes a dedicated connection per transaction, and threaded a Queryable
interface through the services so they accept either a pool or a pooled
client. Both reprocess_artists merge blocks — the artist merge and the
duplicate-album merge — now run inside withTransaction; previously a failure
partway through left artists merged and their tracks unmoved.

integrity.service and cleanup.service get only the constructor type change
here so this commit compiles; their own fixes follow in the next two
commits. cleanup.service's BEGIN/COMMIT-on-a-Pool is therefore still wrong
at this commit and is replaced wholesale by the hard-delete commit.

REVIEW-2026-07-30.md finding 4 (and the concurrency note in finding 3).

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

workers/src/scanner.service.ts

@@ -3,7 +3,7 @@ import { createReadStream } from 'fs';
 import { createHash } from 'crypto';
 import path from 'path';
 import mm from 'music-metadata';
-import { Client as PgClient } from 'pg';
+import type { Queryable } from './db.js';
 import { Queue } from 'bullmq';
 import { MetadataRefreshJob, ArtistSimilarityJob, ArtistImageJob, AlbumCoverJob } from './types.js';
 import { splitArtistNames, parseArtists } from './utils/artist-names.js';
@@ -80,7 +80,7 @@ export class ScannerService {
   private enqueuedArtists = new Set<string>();
   private enqueuedAlbums = new Set<string>();
 
-  constructor(private pgClient: PgClient, private queue: Queue) {}
+  constructor(private pgClient: Queryable, private queue: Queue) {}
 
   async scanDirectory(directory: string) {
     console.log(`[Scanner] Starting scan in: ${directory}`);