fix(api): return empty arrays not null for empty lists (frontend null.length crash)

Root cause: store List* methods used 'var out []T' which stays nil on empty
result sets and serializes to JSON null; the SPA then crashed on .length/.map
(e.g. endpoints.length on the Tasks page right after deploy). Return []T{}
at the source; coerce null->[] on the frontend load sites as defense-in-depth.
Regression test asserts List* are non-nil when empty.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMHQTtnQtQqL8muAXHr9kd

web/src/pages/Endpoints.tsx

@@ -11,7 +11,7 @@ export function Endpoints() {
 
   function reload() {
     listEndpoints()
-      .then(setEndpoints)
+      .then((e) => setEndpoints(e ?? []))
       .catch((e) => setError(String(e.message || e)))
   }
 

web/src/pages/Tasks.tsx

@@ -13,13 +13,13 @@ export function Tasks() {
 
   function reload() {
     listTasks()
-      .then(setTasks)
+      .then((t) => setTasks(t ?? []))
       .catch((e: unknown) => setError(e instanceof Error ? e.message : 'Failed to load tasks'))
   }
 
   useEffect(() => {
     reload()
-    listEndpoints().then(setEndpoints).catch(() => {})
+    listEndpoints().then((e) => setEndpoints(e ?? [])).catch(() => {})
   }, [])
 
   async function submit(e: FormEvent) {