feat: secure resource explanation subjects

src/govoplan_access/backend/manifest.py

@@ -301,6 +301,57 @@ ADMIN_READ_SCOPES = (
 )
 
 ACCESS_DOCUMENTATION: tuple[DocumentationTopic, ...] = (
+    DocumentationTopic(
+        id="access.reference.resource-explanation-subjects",
+        title="Select a user for resource-access diagnostics",
+        summary=(
+            "Resource explanations default to the signed-in user and expose "
+            "other tenant users only when Policy permits the diagnostic."
+        ),
+        body=(
+            "The shared Files and Campaign explanation dialog asks Access for "
+            "the permitted subject list. If Policy is unavailable or the actor "
+            "lacks policy:access_explanation:select_user, Access returns only "
+            "the signed-in active membership and no metadata for other users. "
+            "When Policy permits selection, the picker is limited to active "
+            "users in the current tenant. Every explanation run for another "
+            "user creates audit evidence with the target membership, resource, "
+            "requested action, and policy source. The explanation is diagnostic "
+            "and never grants resource access."
+        ),
+        layer="configured",
+        documentation_types=("admin", "user"),
+        audience=("tenant_admin", "access_admin", "security_reviewer"),
+        order=29,
+        conditions=(
+            DocumentationCondition(
+                required_modules=("access",),
+                any_scopes=(
+                    "admin:users:read",
+                    "admin:roles:read",
+                    "access:membership:read",
+                    "access:role:read",
+                ),
+            ),
+        ),
+        links=(
+            DocumentationLink(
+                label="Permitted explanation subjects API",
+                href="/api/v1/admin/access/resource-explanation/subjects",
+                kind="api",
+            ),
+            DocumentationLink(
+                label="Resource explanation API",
+                href="/api/v1/admin/access/resource-explanation",
+                kind="api",
+            ),
+        ),
+        related_modules=("audit", "campaigns", "files", "policy"),
+        metadata={
+            "kind": "reference",
+            "help_contexts": ["access.resource-explanation.subject"],
+        },
+    ),
     DocumentationTopic(
         id="access.operator.enroll-first-administrator",
         title="Enroll the first production administrator",