Docs: guides in the indexes, the stalest-kind rule, and the changelog

Guides was shipped without an entry in PLUGINS.md or START-HERE, so a site
adopting the product had no way to find out it exists.

BACKUP-KINDS.md described a host's badge as coming from the most recently
confirmed kind. It has not worked that way since the fleet report was reworked:
the freshest revision per (host, kind) is taken, and then the STALEST of those
kinds decides, because reporting the freshest would let a healthy NTLARS backup
mask a part-marker capture that stopped a month ago. Stalest among kinds that
can be JUDGED - a kind never confirmed is unmeasurable, not stale, and treating
it as the worst value made a host with a working backup show no badge at all.

The backups row also still advertised UDC and CMM kinds that do not exist.

api-inventory, openapi.json and PROJECT-MAP regenerated.

docs/api-inventory.json

@@ -535,6 +535,22 @@
         "params": "none",
         "example": "curl http://localhost:5001/api/auditlogs/stats -H \"Authorization: Bearer $JWT\""
       },
+      {
+        "method": "GET",
+        "path": "/api/auditlogs/retention",
+        "auth": "jwt + audit.view",
+        "params": "none",
+        "purpose": "Audit log size (total entries, oldest/newest timestamps) and, when a retention period is configured, how many entries a purge would delete. Drives the Site & Facility retention card so the count is seen before anything is deleted",
+        "example": "curl http://localhost:5001/api/auditlogs/retention -H \"Authorization: Bearer $TOKEN\""
+      },
+      {
+        "method": "POST",
+        "path": "/api/auditlogs/purge",
+        "auth": "jwt + audit.purge",
+        "params": "body: days (optional, defaults to the audit_retentiondays setting), dryrun (optional bool)",
+        "purpose": "Delete audit entries older than the retention window, in committed batches. Requires audit.purge, not merely audit.view. Batch-capped so the request cannot outlive a proxy timeout; an incomplete run returns complete=false with the number still remaining rather than reporting a finished job. Refuses with 400 when no retention period is configured",
+        "example": "curl -X POST http://localhost:5001/api/auditlogs/purge -H \"Authorization: Bearer $TOKEN\" -H 'Content-Type: application/json' -d '{\"days\":365,\"dryrun\":true}'"
+      },
       {
         "method": "GET",
         "path": "/api/plugins",