feat(ui): contribution graph with daily activity heatmap

Add /v1/activity/daily endpoint returning per-day event counts via
generate_series + LEFT JOIN. Frontend renders an SVG contribution
graph with circles colored by quantile-based thresholds. Clicking a
day navigates to /activity/YYYY-MM-DD showing that day's events.

New /activity/:timespan route parses single dates (YYYY-MM-DD) and
ranges (YYYY-MM-DD..YYYY-MM-DD) from the URL to initialize the
activity timeline filter.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/TimelineHome.tsx

@@ -1,4 +1,5 @@
 import { useMemo, useState } from 'react';
+import { useParams } from 'react-router-dom';
 import { useQuery } from '@tanstack/react-query';
 import Col from 'react-bootstrap/Col';
 import Row from 'react-bootstrap/Row';
@@ -11,7 +12,24 @@ import { TimelineEntry } from '../components/TimelineEntry';
 const RANGE_MIN = new Date('2010-01-01T00:00:00Z').getTime();
 const RANGE_MAX = Date.now();
 
+function parseTimespan(timespan?: string): [number, number] | null {
+  if (!timespan) return null;
+  if (timespan.includes('..')) {
+    const [a, b] = timespan.split('..');
+    const from = new Date(a + 'T00:00:00Z').getTime();
+    const to = new Date(b + 'T23:59:59Z').getTime();
+    if (!isNaN(from) && !isNaN(to)) return [from, to];
+  } else {
+    const from = new Date(timespan + 'T00:00:00Z').getTime();
+    const to = new Date(timespan + 'T23:59:59Z').getTime();
+    if (!isNaN(from)) return [from, to];
+  }
+  return null;
+}
+
 export function TimelineHome() {
+  const { timespan } = useParams();
+
   const [enabledSources, setEnabledSources] = useState<Record<Source, boolean>>({
     github: true,
     gitea: true,
@@ -19,6 +37,8 @@ export function TimelineHome() {
     bugzilla: true,
   });
   const [rangeValue, setRangeValue] = useState<[number, number]>(() => {
+    const parsed = parseTimespan(timespan);
+    if (parsed) return parsed;
     const now = Date.now();
     const thirtyDaysAgo = now - 30 * 24 * 60 * 60 * 1000;
     return [thirtyDaysAgo, now];