Add governed source freshness audit

docs/API_CONTRACTS.md

@@ -394,6 +394,24 @@ the existing MapLibre image-overlay path.
 
 List datasets.
 
+### GET `/api/v1/projects/{project_id}/datasets/source-freshness`
+
+Returns one read-only, canonical source-governance report for all persisted
+project datasets. Datasets are grouped by `source_name` (falling back to
+`source`) and classified as a rolling snapshot, annual release, fixed edition,
+scenario, historical archive or local artifact.
+
+Each source item reports dataset/version counts, latest import and observation
+evidence, latest source version, next review date where meaningful, historical
+series availability and local integrity counts for missing DatasetVersions,
+checksum mismatches, missing storage files and size mismatches. Status is one
+of `current`, `due`, `review_required` or `local`.
+
+This endpoint never contacts an external provider, downloads data, mutates a
+Dataset or silently refreshes a publication. Fixed editions and scenarios are
+not marked stale merely because their source date is old. Unknown sources are
+`review_required` until an explicit publication policy is defined.
+
 ### GET `/api/v1/projects/{project_id}/datasets/{dataset_id}`
 
 Return metadata.