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/caddy-auth.caddy

@@ -0,0 +1,19 @@
+# Optional Basic-auth gate for the collage.
+# Use only if you're NOT putting Cloudflare Access in front of the
+# public URL. Replace the bcrypt hash below — generate via:
+#   caddy hash-password --plaintext 'your-pass'
+#
+# Drop this into /etc/caddy/conf.d/ alongside avian.caddy and reload.
+
+(avian_basicauth) {
+    basic_auth {
+        # username   bcrypt-hash
+        avian        $2a$14$REPLACE_WITH_HASH_FROM_caddy_hash-password
+    }
+}
+
+# Example: gate only the public hostname, leave LAN open.
+# https://birds.your-domain.com {
+#     import avian_basicauth
+#     reverse_proxy localhost:80
+# }