fix(map): revert to Leaflet + proxied OSM tiles, add CPE signal to popups

Reverted from MapLibre/PMTiles to Leaflet with nginx-proxied OSM raster
tiles — the MapLibre approach had unresolvable CSP and theme compat
issues. The proxy keeps all browser requests local (no third-party).

Also:
- Add CPE signal strength and parent AP name to fleet summary SQL
  and map popup cards (e.g. "Signal: -62 dBm to ap-shady-north")
- Add .dockerignore to exclude 8GB PMTiles and node_modules from
  Docker build context (was causing 10+ minute builds)
- Configure mailpit SMTP in dev compose

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

frontend/src/lib/api.ts

@@ -809,6 +809,8 @@ export interface FleetDevice {
   tenant_name: string
   client_count: number | null
   avg_signal: number | null
+  cpe_signal: number | null
+  ap_hostname: string | null
 }
 
 export interface SparklinePoint {