Add per-user stats: play/error events + /api/stats + ?stats page

- server: 'play' event on successful stream fetch, 'error' on 404 stream
  failures; new POST /api/events for client-reported playback errors
  (only type=error accepted, fields allowlisted + length-capped)
- server: GET /api/stats?days=N — watch time from position-event pos
  deltas (<=90s cap: bigger jumps are seeks), episodes started/completed
  (>=95% of dur), days active, weekday histogram, top anime/searches,
  sub/dub split, error count
- frontend: ?stats page (cards, weekday bars, most-watched, top
  searches) + 'stats' link on home; playerFail now reports the error
- test: test_stats with synthetic viewing data (deltas, isolation, 422)

static/app.js

@@ -63,6 +63,10 @@ function postHistory(entry) {
   fetch(BASE + "api/history", {method: "POST", headers: {"Content-Type": "application/json", "X-Aniweb": "1"},
     body: JSON.stringify(entry)}).catch(() => {});
 }
+function postEvent(ev) {
+  fetch(BASE + "api/events", {method: "POST", headers: {"Content-Type": "application/json", "X-Aniweb": "1"},
+    body: JSON.stringify(ev)}).catch(() => {});
+}
 const fmtPos = s => { s = Math.floor(s || 0); return `${Math.floor(s / 60)}:${String(s % 60).padStart(2, "0")}`; };
 const timeago = t => {
   if (!t) return "";
@@ -110,6 +114,9 @@ function showHome() {
   if (history.length > 1) {
     const head = el("div", "sec-head");
     head.append(el("h2", null, "Recently played"));
+    const st = el("a", null, "stats \u2192");
+    st.href = BASE + "?stats";
+    head.append(st);
     if (history.length >= 5) {
       const va = el("a", null, "view all \u2192");
       va.href = BASE + "?history";
@@ -323,6 +330,64 @@ function histCard(x) {
   return c;
 }
 
+// ---------- stats (last 30 days, server-computed) ----------
+const fmtMin = m => { m = Math.round(m || 0);
+  return m >= 60 ? `${Math.floor(m / 60)}h ${m % 60}m` : `${m}m`; };
+const statCard = (label, value) => {
+  const c = el("div", "card stat");
+  c.append(el("div", "stat-value", value), el("div", "muted", label));
+  return c;
+};
+async function showStats() {
+  main.innerHTML = "";
+  main.append(el("h2", null, "Stats \u2014 last 30 days"));
+  skel(5);
+  const d = await api("/api/stats?days=30");
+  main.innerHTML = "";
+  main.append(el("h2", null, "Stats \u2014 last 30 days"));
+  const cards = el("div", "grid stats");
+  cards.append(
+    statCard("watch time", fmtMin(d.watch_minutes)),
+    statCard("episodes finished / started", `${d.episodes.completed} / ${d.episodes.started}`),
+    statCard("days active", String(d.days_active)),
+    statCard("sub \u00b7 dub plays", `${d.sub_dub.jpn} \u00b7 ${d.sub_dub.eng}`),
+    statCard("playback errors", String(d.errors)));
+  main.append(cards);
+  const WD = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"];
+  const mx = Math.max(1, ...d.by_weekday);
+  const wk = el("div", "wk");
+  d.by_weekday.forEach((s, i) => {
+    const col = el("div", "wk-col");
+    const b = el("div", "wk-bar");
+    b.style.height = Math.max(3, Math.round(s / mx * 50)) + "px";
+    b.title = fmtMin(s / 60);
+    col.append(b, el("div", "wk-day", WD[i]));
+    wk.append(col);
+  });
+  main.append(wk);
+  if (d.top_anime.length) {
+    const ta = el("section", null);
+    ta.append(el("h2", null, "Most watched"));
+    for (const a of d.top_anime) {
+      const row = el("div", "stat-row");
+      row.append(el("span", "stat-title", a.title),
+                 el("span", "muted", `${fmtMin(a.minutes)} \u00b7 ${a.completed}/${a.started} ep`));
+      const b = el("div", "bar small");
+      const f = el("div", "bar-fill");
+      f.style.width = (a.started ? Math.round(100 * a.completed / a.started) : 0) + "%";
+      b.append(f);
+      row.append(b);
+      ta.append(row);
+    }
+    main.append(ta);
+  }
+  if (d.top_searches.length) {
+    main.append(el("section", null).append(
+      el("h2", null, "Top searches"),
+      el("div", "muted", d.top_searches.join(" \u00b7 "))));
+  }
+}
+
 function markActive(b) {
   for (const x of document.querySelectorAll(".ep")) x.classList.remove("active");
   b.classList.add("active");
@@ -388,6 +453,7 @@ function playerFail(msg) {
   if (!currentEp) return;
   playerBroken = true;
   const { ep_id, ep_no } = currentEp;
+  postEvent({type: "error", detail: msg, ep_id, ep: ep_no, lang});
   playerOverlay("Playback failed: " + msg, () => play(ep_id, ep_no));
 }
 
@@ -541,6 +607,7 @@ window.addEventListener("pagehide", savePos);
       lang: p.get("lang") || "jpn",
     });
     else if (p.has("history")) await showHistory();
+    else if (p.has("stats")) await showStats();
     else showHome();
   } catch (e) { fail(e); }
 })();