map debug + coord backfill, remove email channel, countdown label

- Surface "X/Y passende Wohnungen mit Koordinaten" on the Karte view +
  admin-only "Koordinaten nachladen" button (POST /actions/backfill-coords)
  that geocodes missing flats via Google Maps directly from the web container
- Add googlemaps dep + GMAPS_API_KEY env to web service
- Light console.log in map.js ("rendering N/M markers", "building Leaflet…")
  so the browser DevTools shows what's happening
- Drop e-mail channel from notifications UI, notify dispatcher, and _alert_status;
  coerce legacy 'email' channel rows back to 'ui' on save
- Countdown said "Aktualisierung läuft…" next to "nächste Aktualisierung" →
  shortened to "aktualisiere…"

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/static/app.js

@@ -18,7 +18,7 @@ function fmtCountdown(iso) {
   const ts = Date.parse(iso);
   if (!iso || Number.isNaN(ts)) return "—";
   const secs = Math.floor((ts - Date.now()) / 1000);
-  if (secs <= 0)   return "Aktualisierung läuft…";
+  if (secs <= 0)   return "aktualisiere…";
   if (secs < 60)   return `in ${secs} s`;
   if (secs < 3600) return `in ${Math.floor(secs / 60)} min`;
   return `in ${Math.floor(secs / 3600)} h`;