feat: SQLite persistence, non-blocking inference, modular frontend

Backend:
- Persist alert events and robot delivery counters in SQLite (data/app.db,
  YOLO_DB_PATH override); EventStore keeps its interface, DeliveryStatsStore
  uses read/write-through UPSERT; in-memory fallback remains for standalone
  AlertManager use
- Serve /api/detect from a sync endpoint and serialize YOLO inference with a
  lock, so concurrent requests no longer block the event loop
- Unify single-image and session detection: requests without session_id
  share the fixed single-image session and run full frame confirmation
- Store UTC Z-suffixed timestamps for sortable string comparison; close
  executor and database on shutdown via FastAPI lifespan

Frontend:
- Split app.js into ES modules (main.js + modules/{dom,api,ui,charts,store}
  + views/{dashboard,inspection,events,robots,robotCards}), no build step
- Escape all server data interpolated into innerHTML; guard missing
  event.classes; replace lazy element ID list with memoized qs()
- Remove hardcoded fake stats (trend badge, device donut segment)

Docs:
- Rewrite README: accurate weight policy (only trained best.pt committed,
  no Git LFS), SQLite persistence, single-worker note, data asset inventory
- Align models/pretrained/README.md with actual files; add YOLO_DB_PATH to
  .env.example; add persistence unit tests; ruff clean

frontend/modules/dom.js

@@ -0,0 +1,57 @@
+const elementCache = new Map();
+
+export function qs(selector) {
+  if (!elementCache.has(selector)) {
+    elementCache.set(selector, document.querySelector(selector));
+  }
+  return elementCache.get(selector);
+}
+
+export function escapeHtml(value) {
+  return String(value)
+    .replaceAll("&", "&")
+    .replaceAll("<", "&lt;")
+    .replaceAll(">", "&gt;")
+    .replaceAll('"', "&quot;")
+    .replaceAll("'", "&#039;");
+}
+
+export const VIEW_LABELS = {
+  overview: "系统总览",
+  inspection: "视频巡检",
+  events: "告警中心",
+  robots: "消息机器人",
+  risks: "风险台账",
+  settings: "系统设置",
+};
+
+export const STATUS_LABELS = {
+  pending: "待处置",
+  acknowledged: "已确认",
+  resolved: "已解决",
+};
+
+const CLASS_LABELS = { fire: "火焰", smoke: "烟雾" };
+
+export function classLabel(name) {
+  return CLASS_LABELS[name] || escapeHtml(String(name));
+}
+
+export function formatDate(value) {
+  if (!value) return "--";
+  return new Date(value).toLocaleString("zh-CN", { hour12: false });
+}
+
+export function formatTime(seconds) {
+  if (!Number.isFinite(seconds)) return "00:00";
+  const minutes = Math.floor(seconds / 60);
+  const remaining = Math.floor(seconds % 60);
+  return `${String(minutes).padStart(2, "0")}:${String(remaining).padStart(2, "0")}`;
+}
+
+export function enabledChannelNames(channels = {}) {
+  const names = [];
+  if (channels.wechat) names.push("企业微信");
+  if (channels.feishu) names.push("飞书");
+  return names;
+}