Add owner/admin-only log retention deletion to Settings

New api/logretention package: GET /logs/retention/preview and
DELETE /logs/retention, both gated to RoleAdmin (Owner satisfies it
too), issue purpose-built parameterized statements against ClickHouse's
logs table (a count and a synchronous ALTER TABLE ... DELETE mutation)
rather than routing through querylang/executor's SELECT-only SQLRunner.

Settings gets a new "Log retention" section, visible only to an owner
or admin, that previews how many records a chosen age cutoff would
remove before showing an explicit confirm/cancel panel -- no delete
happens without that second step.

Scoped to core's single-tenant ClickHouse table; enterprise/'s
per-tenant routing and Tantivy's lack of a bulk-delete primitive are
disclosed gaps in api/logretention/store.go's doc comment, not silently
assumed to already work.

web/src/lib/api.ts

@@ -482,6 +482,22 @@ export function setUserRole(id: string, role: string): Promise<LocalUser> {
 	});
 }
 
+// --- log retention (owner/admin only, see api/logretention) -----------
+
+export type LogRetentionPreview = { count: number; cutoff: string };
+export type LogRetentionDeleteResult = { deleted_count: number; cutoff: string };
+
+export function previewLogDeletion(olderThanHours: number): Promise<LogRetentionPreview> {
+	return request(`/logs/retention/preview?older_than_hours=${olderThanHours}`, { credentials: 'include' });
+}
+
+export function deleteLogsOlderThan(olderThanHours: number): Promise<LogRetentionDeleteResult> {
+	return request(`/logs/retention?older_than_hours=${olderThanHours}`, {
+		method: 'DELETE',
+		credentials: 'include'
+	});
+}
+
 // --- alerting ---------------------------------------------------------
 
 export type ConditionType = 'threshold' | 'absence';