Throw on wrong-field-name parse failures instead of silent zero-point success

Spot is now confirmed subscribed to hourly/three-hourly too (settings'
met_office test matrix), so those parsers are genuinely exercised for the
first time -- previously a wrong field name would silently produce an
empty points array while still reporting the tier as a 'met_office'
success. assertParsed() throws when raw entries came back but none parsed
a valid temp_c, so a field-name mismatch now correctly falls back to
Open-Meteo (and logs an error) instead of quietly showing an empty today
strip.

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

backend/src/lib/weather-fetch.js

@@ -62,12 +62,26 @@ async function fetchMetOfficeTimestep(timestep, lat, lon, apiKey) {
   return series
 }
 
+// Throws if the raw response had entries but every one of them failed to parse a
+// valid temp_c — that's not "no data for this range" (which is a legitimate empty
+// array, not an error), it's "the field names are wrong". Confirmed live 2026-08-25
+// that Spot is actually subscribed to all three endpoints (settings' met_office
+// test route), so hourly/three-hourly parsing is now genuinely exercised for the
+// first time — this turns a silent wrong-field-name bug (reported as "success"
+// with zero points) into a real failure that falls back to Open-Meteo instead.
+function assertParsed(rawCount, points, label) {
+  if (rawCount > 0 && points.length === 0) {
+    throw new Error(`Met Office ${label}: got ${rawCount} raw entries but parsed 0 valid points — check field names`)
+  }
+  return points
+}
+
 // Every hour the endpoint returns, not just today — the "tap a Coming up card"
 // hourly-detail modal needs this for tomorrow/day-after (and, where available,
 // days 4-7) too.
 async function fetchMetOfficeHourly(lat, lon, apiKey) {
   const series = await fetchMetOfficeTimestep('hourly', lat, lon, apiKey)
-  return series
+  const points = series
     .map(p => ({
       resolution: 'hourly',
       ts: p.time,
@@ -81,12 +95,13 @@ async function fetchMetOfficeHourly(lat, lon, apiKey) {
       source: 'met_office',
     }))
     .filter(p => typeof p.temp_c === 'number')
+  return assertParsed(series.length, points, 'hourly')
 }
 
 async function fetchMetOfficeThreeHourly(lat, lon, apiKey, dayKeys) {
   const series = await fetchMetOfficeTimestep('three-hourly', lat, lon, apiKey)
-  return series
+  const dayRows = series.filter(p => dayKeys.includes(londonDateKey(p.time)))
-    .filter(p => dayKeys.includes(londonDateKey(p.time)))
+  const points = dayRows
     .map(p => ({
       resolution: '3hourly',
       ts: p.time,
@@ -100,6 +115,7 @@ async function fetchMetOfficeThreeHourly(lat, lon, apiKey, dayKeys) {
       source: 'met_office',
     }))
     .filter(p => typeof p.temp_c === 'number')
+  return assertParsed(dayRows.length, points, 'three-hourly')
 }
 
 // Returns both the daily tier's points AND astro (sunrise/sunset) — the daily
@@ -108,9 +124,8 @@ async function fetchMetOfficeThreeHourly(lat, lon, apiKey, dayKeys) {
 // openMeteoAstroPoints below).
 async function fetchMetOfficeDaily(lat, lon, apiKey, skipKeys, astroKeys) {
   const series = await fetchMetOfficeTimestep('daily', lat, lon, apiKey)
-  const dailyPoints = series
+  const dayRows = series.filter(p => !skipKeys.has(londonDateKey(p.time))).slice(0, 4)
-    .filter(p => !skipKeys.has(londonDateKey(p.time)))
+  const dailyPoints = assertParsed(dayRows.length, dayRows
-    .slice(0, 4)
     .map(p => ({
       resolution: 'daily',
       ts: p.time,
@@ -123,7 +138,7 @@ async function fetchMetOfficeDaily(lat, lon, apiKey, skipKeys, astroKeys) {
       icon: iconFromMetOffice(p.daySignificantWeatherCode),
       source: 'met_office',
     }))
-    .filter(p => typeof p.temp_c === 'number')
+    .filter(p => typeof p.temp_c === 'number'), 'daily')
   const astroPoints = series
     .filter(p => astroKeys.includes(londonDateKey(p.time)))
     .map(p => ({ date: londonDateKey(p.time), sunrise: p.sunrise ?? null, sunset: p.sunset ?? null }))