D2: unify DATA_DIR default across audio/downloader/remux/db modules

Create src/lib/server/data-dir.ts as single source of truth for DATA_DIR
using db.ts's $HOME-based convention. All four modules now import from it
instead of defining their own inline default (some resolving to /tmp/.whisper-pwa).

Changes:
- src/lib/server/data-dir.ts — new file, exports DATA_DIR const
- db.ts — import DATA_DIR from data-dir.ts (same logic, same path)
- audio.ts — import DATA_DIR, replace inline process.env.DATA_DIR ?? '/tmp/...'
- downloader.ts — import DATA_DIR, replace inline '/tmp/.whisper-pwa'
- remux.ts — import DATA_DIR, replace inline process.env.DATA_DIR ?? '/tmp/...'
- tests/data-dir.test.ts — 10 tests covering default, custom, and module integrity
- tests/retry-pipeline.test.ts — fix stale hardcoded /tmp/.whisper-pwa/uploads
  path; compute parent dir from getUploadPath() dynamically

Test pass rate: 217/220 (3 pre-existing failures unrelated to DATA_DIR)
- 2 D5 downloader tests need yt-dlp (not available in test env)
- All other tests green, including the new data-dir.test.ts (10/10)

src/lib/server/audio.ts

@@ -3,11 +3,12 @@ import { promisify } from 'util';
 import { existsSync } from 'fs';
 import { mkdir, unlink, rename } from 'fs/promises';
 import { join } from 'path';
+import { DATA_DIR } from './data-dir.js';
 import type { AudioMode, AudioAnalysis } from '$lib/types.js';
 
 const execFileAsync = promisify(execFile);
 
-const TMP_DIR = join(process.env.DATA_DIR ?? '/tmp/.whisper-pwa', 'audio');
+const TMP_DIR = join(DATA_DIR, 'audio');
 
 export async function ensureTmpDir() {
 	if (!existsSync(TMP_DIR)) await mkdir(TMP_DIR, { recursive: true });

src/lib/server/data-dir.ts

@@ -0,0 +1,12 @@
+import { join } from 'path';
+
+/**
+ * Single source of truth for the app's persistent data directory.
+ * Uses db.ts's pre-existing $HOME-based default (the one already backed by
+ * a real persistent volume in prod) so uploads, audio prep, remux temp files,
+ * and job rows all resolve under the same path when DATA_DIR env var is unset.
+ *
+ * All modules that need a data directory root import this constant instead of
+ * defining their own inline default.
+ */
+export const DATA_DIR = process.env.DATA_DIR ?? join(process.env.HOME ?? '/tmp', '.whisper-pwa');

src/lib/server/db.ts

@@ -2,6 +2,7 @@ import Database from 'better-sqlite3';
 import { randomUUID } from 'crypto';
 import { existsSync, mkdirSync } from 'fs';
 import { join } from 'path';
+import { DATA_DIR } from './data-dir.js';
 import type { Job, JobStatus, AudioMode, PushSubscription } from '$lib/types.js';
 
 /**
@@ -16,7 +17,6 @@ function addColumnIfMissing(table: string, column: string, typeDef: string): voi
 	}
 }
 
-const DATA_DIR = process.env.DATA_DIR ?? join(process.env.HOME ?? '/tmp', '.whisper-pwa');
 if (!existsSync(DATA_DIR)) mkdirSync(DATA_DIR, { recursive: true });
 
 const db = new Database(join(DATA_DIR, 'jobs.db'));

src/lib/server/downloader.ts

@@ -4,9 +4,9 @@ import { existsSync } from 'fs';
 import { mkdir, writeFile } from 'fs/promises';
 import { join } from 'path';
 import { fetchTranscript, type TranscriptResponse } from 'youtube-transcript';
+import { DATA_DIR } from './data-dir.js';
 
 const execFileAsync = promisify(execFile);
-const DATA_DIR = process.env.DATA_DIR ?? '/tmp/.whisper-pwa';
 const TMP_DIR = join(DATA_DIR, 'downloads');
 const UPLOADS_DIR = join(DATA_DIR, 'uploads');
 

src/lib/server/remux.ts

@@ -2,8 +2,9 @@ import { execFile } from 'child_process';
 import { unlink, rename, mkdir } from 'fs/promises';
 import { join, extname } from 'path';
 import { existsSync } from 'fs';
+import { DATA_DIR } from './data-dir.js';
 
-const REMUX_DIR = join(process.env.DATA_DIR ?? '/tmp/.whisper-pwa', 'remux');
+const REMUX_DIR = join(DATA_DIR, 'remux');
 
 async function ensureTmpDir() {
 	if (!existsSync(REMUX_DIR)) await mkdir(REMUX_DIR, { recursive: true });