Implement analytics feature

Add server-side visit analytics collection, a public-page ping endpoint,
and a full-screen AnalyticsView for admins.

Backend:
- Add pagerite/analytics.py: Analytics/Visit model, Store, and persistence
- Wire /_a ping endpoint and GET /_api/analytics into pagerite/app.py

Frontend:
- Add full-screen AnalyticsView with visitor charts and transition map
- Add VisitorCharts and TransitionGraph subcomponents
- Add analytics JS helpers in frontend/src/analytics/
- Send navigation pings from frontend/src/pagerite.js
- Mount AnalyticsView from frontend/src/main.js
- Document the feature in docs/analytics.md and update AGENTS.md

frontend/src/TransitionGraph.vue

@@ -0,0 +1,100 @@
+<script setup>
+/**
+ * Radial transition map filtered to the selected time range.
+ *
+ * The server only stores an all-time transition aggregate, so this component
+ * derives time-filtered transitions from the visits list (which has start
+ * timestamps) and filters the view counts to the same window.
+ */
+import { computed } from 'vue'
+import { rangeWindow } from './analytics/time.js'
+import {
+  TNODE_R,
+  buildTransitionGraph,
+  buildTransitionsFromVisits,
+  filterViewsByRange,
+} from './analytics/transitions.js'
+
+const props = defineProps({
+  data: { type: Object, default: null },
+  range: { type: String, required: true },
+  pageTree: { type: Array, default: null },
+})
+const emit = defineEmits(['close'])
+
+const window = computed(() => rangeWindow(props.range))
+
+const filteredData = computed(() => {
+  if (!props.data) return null
+  const { t0, t1 } = window.value
+  return {
+    transitions: buildTransitionsFromVisits(props.data.visits, t0, t1),
+    views: filterViewsByRange(props.data.views, t0, t1),
+  }
+})
+
+const graph = computed(() =>
+  filteredData.value
+    ? buildTransitionGraph(filteredData.value, props.pageTree)
+    : null,
+)
+</script>
+
+<template>
+  <section v-if="graph">
+    <svg class="tmap" :viewBox="`${graph.bounds.x0} ${graph.bounds.y0} ${graph.bounds.x1 - graph.bounds.x0} ${graph.bounds.y1 - graph.bounds.y0}`"
+         role="img" aria-label="map of transitions between pages">
+      <path v-for="(a, i) in graph.arcs" :key="'a' + i"
+            :d="a.d" class="tarc" />
+      <path v-for="(e, i) in graph.edges" :key="'e' + i"
+            :d="e.d" class="tconn">
+        <title>{{ e.title }}</title>
+      </path>
+      <g v-for="n in graph.nodes" :key="n.path">
+        <a :href="n.path" :title="n.title" @click="emit('close')">
+          <circle :cx="n.x" :cy="n.y" :r="TNODE_R" class="tnode" />
+          <text :x="n.x" :y="n.y - 2" class="tnodeslug">{{ n.label }}</text>
+          <text :x="n.x" :y="n.y + 12" class="tnodecount">{{ n.views }}</text>
+        </a>
+      </g>
+    </svg>
+  </section>
+</template>
+
+<style scoped>
+/* Transition map: radial graph of internal page-to-page transitions. */
+.tmap {
+  display: block;
+  width: 100%;
+  max-width: 36rem;
+  margin: 0 auto;
+}
+.tmap .tconn {
+  fill: var(--accent);
+  opacity: 0.4; /* uniform, not strength-encoded: width carries that */
+}
+.tmap .tarc {
+  fill: none;
+  stroke: var(--line);
+  stroke-width: 1;
+}
+.tmap .tnode {
+  fill: var(--bg, Canvas);
+  stroke: var(--accent);
+  stroke-width: 1.5;
+}
+.tmap .tnodeslug {
+  fill: var(--text);
+  font-size: 11px;
+  text-anchor: middle;
+}
+.tmap a { cursor: pointer; }
+.tmap a:hover .tnodeslug { fill: var(--accent); }
+.tmap .tnodecount {
+  fill: var(--muted);
+  font-size: 10px;
+  text-anchor: middle;
+}
+
+section { margin-top: 1.8rem; }
+</style>