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/fixtures/gs1_mapping.json

@@ -0,0 +1,14 @@
+{
+  "06901234567892": {
+    "name": "示例矿泉水 550ml",
+    "brand": "示例品牌",
+    "manufacturer": "示例饮品有限公司",
+    "gpc_brick_code": "10000224",
+    "country_of_origin": "China",
+    "net_content_value": 550,
+    "net_content_unit": "ml"
+  },
+  "00000000000000": {
+    "brand": ""
+  }
+}