AvianVisitors: BirdNET-Pi collage frontend overlay

Adds avian/ on top of upstream BirdNET-Pi:
- frontend/    static collage UI (mask-packed bird tiles, sized by detection count)
- assets/      ~450 bundled kachō-e illustrations + cutouts + binary masks
- api/         PHP shims for recent/lifelist/firstseen/timeseries JSON
- scripts/     installer + Gemini pregen + editable prompt template
- caddy/       snippet mounting /collage on existing Caddy
- forwarding/  optional HA / MQTT / Cloudflare configs

Default install: http://birdnet.local/collage/ with no auth.

Upstream BirdNET-Pi README preserved at README.upstream.md.

avian/forwarding/avian-mqtt.service

@@ -0,0 +1,12 @@
+[Unit]
+Description=AvianVisitors MQTT bridge
+After=network-online.target
+
+[Service]
+Type=simple
+ExecStart=/usr/bin/python3 /home/%i/avian-mqtt.py
+Restart=on-failure
+RestartSec=10
+
+[Install]
+WantedBy=multi-user.target