feat: serve melody .bsm binaries over plain HTTP instead of Firebase Storage

ESP32 devices can't spare the 40KB+ RAM a TLS client needs, so Firebase
Storage's HTTPS-only download URLs were blocking melody downloads. Binaries
are now written to local disk (./data/melody_binaries) and served through a
new unauthenticated /api/melodies/download/{pid} route, exposed publicly on
a separate melodies.bellsystems.net vhost (plain HTTP, no TLS) so the main
console domain can stay HTTPS-only with no exceptions. Preview audio still
uses Firebase Storage since it's only ever fetched by the HTTPS admin UI.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/config.py

@@ -33,6 +33,8 @@ class Settings(BaseSettings):
     built_melodies_storage_path: str = "./storage/built_melodies"
     firmware_storage_path: str = "./storage/firmware"
     flash_assets_storage_path: str = "./storage/flash_assets"
+    melody_binaries_storage_path: str = "./storage/melody_binaries"
+    melody_download_base_url: str = "http://melodies.bellsystems.net/download"
 
     # Email (Resend)
     resend_api_key: str = "re_placeholder_change_me"