feat: harden datasource materialization payloads

src/govoplan_datasources/backend/tabular.py

@@ -111,11 +111,33 @@ def fingerprint_rows(
 
 
 def encoded_size(rows: Sequence[Mapping[str, object]]) -> int:
-    return len(
-        json.dumps(rows, sort_keys=True, separators=(",", ":"), default=str).encode(
-            "utf-8"
-        )
-    )
+    return len(_encoded_rows(rows))
+
+
+def payload_checksum(rows: Sequence[Mapping[str, object]]) -> str:
+    return hashlib.sha256(_encoded_rows(rows)).hexdigest()
+
+
+def payload_row_checksum(row: Mapping[str, object]) -> str:
+    return hashlib.sha256(_encoded_row(row)).hexdigest()
+
+
+def _encoded_rows(rows: Sequence[Mapping[str, object]]) -> bytes:
+    return json.dumps(
+        rows,
+        sort_keys=True,
+        separators=(",", ":"),
+        default=str,
+    ).encode("utf-8")
+
+
+def _encoded_row(row: Mapping[str, object]) -> bytes:
+    return json.dumps(
+        row,
+        sort_keys=True,
+        separators=(",", ":"),
+        default=str,
+    ).encode("utf-8")
 
 
 def field_payload(field: DatasourceField) -> dict[str, object]:
@@ -194,5 +216,7 @@ __all__ = [
     "fingerprint_rows",
     "infer_schema",
     "normalize_rows",
+    "payload_checksum",
+    "payload_row_checksum",
     "parse_csv_rows",
 ]