M4: ingestion management (incremental, GS1 supplement, dedup/conflict, quality, scheduler)

- OFF incremental fetch via search API + persistent watermark (ingest_state, migration 0004)
- GS1 barcode supplement adapter (offline mapping + GS1-style API) filling only gaps with field-level provenance
- Non-GTIN dedup with canonical selection + merge_log; field-level conflict resolution (source trust > recency)
- Quality scoring (0.4 completeness + 0.3 source trust + 0.2 multi-source + 0.1 freshness) wired into load/merge
- Jobs: update_off, dedup, schedule; docs/ingestion-management.md
- 19 new tests (pure + DB-integration), ruff clean

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

ingestion/tests/test_state.py

@@ -0,0 +1,16 @@
+from opengoods.etl.state import get_watermark, set_watermark
+
+
+def test_watermark_roundtrip_and_monotonic(db_conn):
+    src = "test-source"
+    assert get_watermark(db_conn, src) == 0
+
+    set_watermark(db_conn, src, 100, stats={"loaded": 1})
+    assert get_watermark(db_conn, src) == 100
+
+    # A lower watermark must not rewind progress.
+    set_watermark(db_conn, src, 50)
+    assert get_watermark(db_conn, src) == 100
+
+    set_watermark(db_conn, src, 150)
+    assert get_watermark(db_conn, src) == 150