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/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');