README: document audio fallback library and download-sunrise-sounds.py usage

https://claude.ai/code/session_01C4jbd3waXG3eKZYbGUjLUQ

README.md

@@ -158,6 +158,28 @@ If your camera has a microphone, sky-cam can mix a natural-speed 10-second audio
 
 The audio is recorded during the same window as the images, stored alongside them, and deleted automatically after being mixed into the final video. No audio library or AI required — it's the real sound from your camera.
 
+### Audio fallback library (optional)
+
+If the camera has no mic, or audio capture fails, `daily_sunrise_video.sh` picks a random ambient sound from `sunrise-sounds/` instead.  The library is organised into 11 weather/season folders (`clear-spring`, `rain`, `thunder`, `windy`, etc.) — populate it once with:
+
+```bash
+# Get a free API key at https://freesound.org/apiv2/apply/
+python3 download-sunrise-sounds.py --api-key YOUR_KEY
+```
+
+Default: ~275 CC-licensed 128 kbps MP3 previews (~25 per category).  Re-run any time to top up:
+
+```bash
+python3 download-sunrise-sounds.py --api-key YOUR_KEY --per-category 40
+```
+
+Attribution data for every file is written to `sunrise-sounds/manifest.json`.
+
+**Priority order for sunrise audio:**
+1. Camera mic recording (`sunrise-audio.m4a`) — real ambient sound at actual sunrise
+2. Random file from `sunrise-sounds/` library — weather/season matched in a future update
+3. No audio — overlay-only video (always produced regardless)
+
 ---
 
 ## Manual operations