Add plain-language summary + umbrella tip for today

Neither Met Office's Global Spot API nor Open-Meteo return a narrative text
summary -- both are purely numeric/coded. New lib/summary.js derives one
rule-based from today's already-fetched hourly points (icon + precip_prob),
no extra API call, no LLM:
- Headline: "Scattered showers today", "Rain likely later", "Heavy rain
  today", "Dry today", etc. -- compares rain distribution across the whole
  day vs specifically the hours still ahead of "now".
- Note: "Thunderstorms possible", appended when a thunder icon shows up.
- Umbrella nudge: shown when there's a real rain chance now or later today.

Computed fresh per-request (not cached at fetch time), since "later" means
later than whoever's looking at the screen right now, not later than the
last hourly refresh.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/src/lib/summary.js

@@ -0,0 +1,68 @@
+// Plain-language summary of today's forecast ("Scattered showers today",
+// "Rain likely later", "Don't forget your umbrella"...). Neither Met Office's Global
+// Spot API nor Open-Meteo return anything like this — both are purely numeric/coded
+// (temps, weather codes, precip%) — so this is generated from today's already-fetched
+// hourly points (icon + precip_prob), rule-based, no extra API call and no LLM.
+//
+// Computed fresh per-request in routes/forecast.js (not stored/cached at fetch time),
+// since "later" is relative to whoever's looking at the screen right now, not to when
+// the hourly data was last fetched.
+const RAIN_ICONS = new Set(['drizzle', 'rain-light', 'rain-heavy', 'sleet', 'hail'])
+const SNOW_ICONS = new Set(['snow-light', 'snow-heavy'])
+
+// summarizeToday(points, nowHour) — `points` is today's hourly forecast_points rows
+// (already icon-normalized), `nowHour` is 0-23 in Europe/London. Returns null if
+// there's nothing to summarize (no data yet).
+export function summarizeToday(points, nowHour) {
+  if (!points.length) return null
+
+  const isRain = p => RAIN_ICONS.has(p.icon)
+  const isSnow = p => SNOW_ICONS.has(p.icon)
+  const isThunder = p => p.icon === 'thunder'
+  const isHeavyRain = p => p.icon === 'rain-heavy'
+
+  const rainPoints = points.filter(isRain)
+  const snowPoints = points.filter(isSnow)
+  const thunderPoints = points.filter(isThunder)
+  const rainFraction = rainPoints.length / points.length
+  const snowFraction = snowPoints.length / points.length
+
+  const laterPoints = points.filter(p => hourOf(p) >= nowHour)
+  const earlierPoints = points.filter(p => hourOf(p) < nowHour)
+  const laterWetFraction = laterPoints.length
+    ? laterPoints.filter(p => isRain(p) || isThunder(p)).length / laterPoints.length
+    : 0
+  const earlierWetCount = earlierPoints.filter(p => isRain(p) || isThunder(p)).length
+
+  let headline
+  if (snowFraction > 0.3) {
+    headline = 'Snow expected today'
+  } else if (rainPoints.length === 0 && thunderPoints.length === 0) {
+    headline = 'Dry today'
+  } else if (rainPoints.some(isHeavyRain) && rainFraction > 0.5) {
+    headline = 'Heavy rain today'
+  } else if (rainFraction > 0.6) {
+    headline = 'Rain likely for much of the day'
+  } else if (laterWetFraction > 0.4 && earlierWetCount === 0) {
+    headline = 'Rain likely later'
+  } else if (rainFraction > 0) {
+    headline = 'Scattered showers today'
+  } else {
+    headline = 'Thunderstorms possible'
+  }
+
+  // Append rather than replace the headline when thunder shows up alongside a
+  // rain pattern that already earned its own headline above.
+  const note = thunderPoints.length > 0 && headline !== 'Thunderstorms possible'
+    ? 'Thunderstorms possible'
+    : null
+
+  // Umbrella nudge: real rain chance either right now or coming up later today.
+  const umbrella = laterWetFraction > 0.25 || rainFraction > 0.25
+
+  return { headline, note, umbrella }
+}
+
+function hourOf(point) {
+  return parseInt(new Date(point.ts).toLocaleString('en-GB', { timeZone: 'Europe/London', hour: '2-digit', hour12: false }))
+}