AvianVisitors hardening pass — install via fork's newinstaller

Restructures the install so users run ONE command. The fork's
newinstaller.sh clones this repo (instead of upstream Nachtzuster) and
runs BirdNET-Pi's installer. install_services.sh symlinks the avian/
overlay into the Caddy site root, so http://birdnet.local/avian/ comes
up after reboot with no Caddy config needed.

Reviewer-flagged fixes:

Frontend (avian/frontend/apt.js)
- Switch all API paths from /api/*.json (which didn't route) to relative
  action-based URLs (./api/birdnet-api.php?action=recent etc.).
- Fetch ./masks.json + ./dims.json relatively (were /avian/*.json).
- Replace every innerHTML user-content sink with createElement +
  textContent + setAttribute — species labels are user-editable in
  BirdNET-Pi, so untrusted.
- Add esc() helper for the few remaining innerHTML attribute contexts.
- fetchJson throws on non-200 (was silently parsing 404 HTML as JSON).
- Defensive null-check in loadMask if MASKS hasn't loaded.

Backend (avian/api/*.php)
- birdnet-api.php now derives DB_PATH from dirname(__DIR__, 3) so any
  BirdNET-Pi install user works (no hardcoded /home/birdnet).
- recording/spectrogram/cutout use getenv('HOME') for the same reason.
- cutout.php reworked as the canonical image resolver: bundled
  illustration → bundled cutout → cached rembg → fresh
  Wikipedia+rembg, with the binomial regex guard and atomic rename
  on cache write. Wikipedia URL host pinned to wikimedia.org /
  wikipedia.org to block SSRF.
- Stale 'X-BirdNET-Proxy-Token' auth claims removed everywhere.

Pregen (avian/scripts/pregen.py)
- Gemini key moved from URL query to x-goog-api-key header.
- Model bumped to gemini-2.5-flash-image-preview.
- Bounded retry on 429 + 5xx with Retry-After honoured.
- Default --sleep raised from 1s to 4s (under free-tier RPM).
- Unified parser handles |/_/comma in all three input modes.
- responseModalities now [TEXT, IMAGE] (was IMAGE-only).
- --poses validated against POSES keys.
- ASCII [ok]/[fail] markers (was ✓/✗ — Windows console crash).
- Surfaces finishReason + blockReason on safety blocks.

Prompt template
- 'Two placeholders' typo → three.
- 'warm paper' / 'transparent background' contradiction resolved.

Forwarding
- mqtt-bridge.py: paho-mqtt 2.x CallbackAPIVersion compat shim.
- avian-mqtt.service: dropped broken %i, set User=birdnet + %h path.
- forwarding/README.md uses the actual action-based URLs.

README
- One-line install via curl … newinstaller.sh.
- No more 'install BirdNET-Pi separately' step (our fork does it).
- License section uses absolute github.com links.
- bird.onethreenine.net replaces the not-yet-existent project writeup.
- docs/thumb.png replaced with the 24-bird Twitter capture.

Removed
- avian/caddy/ and avian/scripts/install.sh — no longer needed; the
  overlay symlink in install_services.sh handles everything.

scripts/install_services.sh

@@ -69,6 +69,8 @@ create_necessary_dirs() {
   sudo -u ${USER} ln -fs $my_dir/include_species_list.txt $my_dir/scripts
   sudo -u ${USER} ln -fs $my_dir/whitelist_species_list.txt $my_dir/scripts
   sudo -u ${USER} ln -fs $my_dir/homepage/* ${EXTRACTED}
+  # AvianVisitors overlay — served at http://birdnet.local/avian/
+  [ -d $my_dir/avian ] && sudo -u ${USER} ln -fs $my_dir/avian ${EXTRACTED}/avian
   sudo -u ${USER} ln -fs $my_dir/model/labels.txt ${my_dir}/scripts
   sudo -u ${USER} ln -fs $my_dir/scripts ${EXTRACTED}
   sudo -u ${USER} ln -fs $my_dir/scripts/play.php ${EXTRACTED}