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/conftest.py

@@ -0,0 +1,31 @@
+"""Shared test fixtures.
+
+`db_conn` yields a psycopg connection inside a transaction that is rolled back
+after each test, so DB tests stay isolated and leave no residue. Tests are
+skipped automatically when no database is reachable or M4 migrations are not
+applied (e.g. local runs without docker).
+"""
+
+from __future__ import annotations
+
+import psycopg
+import pytest
+
+from opengoods.etl.load import default_dsn
+
+
+@pytest.fixture()
+def db_conn():
+    try:
+        conn = psycopg.connect(default_dsn(), connect_timeout=3)
+    except psycopg.OperationalError as exc:  # pragma: no cover - env dependent
+        pytest.skip(f"no database available: {exc}")
+    has_state = conn.execute("SELECT to_regclass('public.ingest_state') IS NOT NULL").fetchone()[0]
+    if not has_state:
+        conn.close()
+        pytest.skip("M4 migrations not applied")
+    try:
+        yield conn
+    finally:
+        conn.rollback()
+        conn.close()