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 });