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/settings.py

@@ -63,3 +63,6 @@ SMTP_STARTTLS: bool = getenv("SMTP_STARTTLS", "true").lower() in ("true", "1", "
 
 # --- App URL (used to build links in notifications) ---------------------------
 PUBLIC_URL: str = getenv("PUBLIC_URL", "https://flat.lab.moritz.run")
+
+# --- Google Maps (used for one-shot coord backfill via admin action) ----------
+GMAPS_API_KEY: str = getenv("GMAPS_API_KEY", "")