refactor: named preprocess timeout and single meta read

Add PREPROCESS_TIMEOUT config (default 120s). Expose read_meta() and use
one read per job in list_jobs() instead of read_status + read_manifest.

Co-authored-by: Cursor <cursoragent@cursor.com>

.env.example

@@ -17,6 +17,7 @@ FILTER_TIMEOUT=120
 FILTER_TIMEOUT_LONG=300
 MAX_FILTER_ATTEMPTS=8
 MAX_EDGE_PX=2000
+PREPROCESS_TIMEOUT=120
 # birefnet-general OOMs on the ~8GB box; u2net + alpha ≈ `rembg i -a`
 REMBG_MODEL=u2net
 REMBG_ALPHA=1