Add governed Statbel edition probe

backend/README.md

@@ -1444,8 +1444,8 @@ docker exec geointel python /app/scripts/audit_source_freshness.py \
 Use `--fail-on due` to make a planned review date fail automation, or
 `--fail-on never` for reporting only. The command never starts a refresh.
 
-An operator can explicitly add the official GRB, orthophoto and ALZ edition
-check:
+An operator can explicitly add the official GRB, orthophoto, Statbel and ALZ
+edition check:
 
 ```bash
 docker exec geointel python /app/scripts/audit_source_freshness.py \
@@ -1459,14 +1459,18 @@ Use `--refresh-catalogs` to bypass the 15-minute in-memory cache and
 `--fail-on-catalog` only when temporary official-provider unavailability must
 fail an operator job. This path reads bounded WFS/WMS capabilities and their
 fixed ISO 19139 metadata records. It confirms GRB `GBG`, `WBN`, `WGO`, `ADP`
-and orthophoto `Ortho`, `Vliegdagcontour`. It also reads the exact official ALZ
+and orthophoto `Ortho`, `Vliegdagcontour`. It parses the exact official Statbel
+DCAT Turtle catalog to identify the latest population-by-statistical-sector
+year, landing page, license and allowed distribution identities. It never
+follows those ZIP/XLSX links. It also reads the exact official ALZ
 publication page and validates only allowlisted archive-link identities. It
 never requests feature, raster or ALZ ZIP content. ALZ v1/v2 campaign snapshots
 remain provisional; only a v3 publication is compared with a local definitive
 historical edition.
 
 Runtime controls are `SOURCE_CATALOG_PROBE_ENABLED`,
-`SOURCE_CATALOG_GRB_WFS_URL`, `SOURCE_CATALOG_ALZ_RELEASE_URL`,
+`SOURCE_CATALOG_GRB_WFS_URL`, `SOURCE_CATALOG_STATBEL_DCAT_URL`,
+`SOURCE_CATALOG_STATBEL_MAX_RESPONSE_MB`, `SOURCE_CATALOG_ALZ_RELEASE_URL`,
 `SOURCE_CATALOG_PROBE_TIMEOUT_SECONDS`, `SOURCE_CATALOG_PROBE_MAX_RESPONSE_MB` and
 `SOURCE_CATALOG_PROBE_CACHE_TTL_SECONDS`. Metadata links remain restricted to
 the official HTTPS CSW path on `metadata.vlaanderen.be` even when an operator
@@ -1474,6 +1478,12 @@ overrides the capabilities endpoint. The ALZ release URL is fail-closed to the
 exact HTTPS host/path and cannot be redirected to another page or download
 host.
 
+The Statbel catalog has a separate 5 MiB default response bound. Population
+year, sector-geometry year and REDEGEO layout remain separate concepts: the
+2025 population release uses the new layout, while the concurrently published
+old layout is transition evidence only. The existence of 2026 sector geometry
+does not imply a 2026 population-by-sector release.
+
 ## Governed regional GRB refresh
 
 `GET /api/v1/projects/{project_id}/datasets/grb-refresh-plan` combines the