fix: share soundcard between darkice and ISR via ALSA dsnoop

hw:0,0 is an exclusive ALSA device — darkice holding it caused arecord
to fail silently (stderr was /dev/null), leaving all recordings at 0 bytes
with no errors in the log.

asound.conf: defines a dsnoop virtual device 'shared_mic' that opens
hw:0,0 once and lets multiple processes capture simultaneously.

docker-compose.yml: mount asound.conf into the container as
/etc/asound.conf; add ipc: host so the container shares the host IPC
namespace (dsnoop uses System V shared memory which does not cross the
container IPC boundary without this).

config.example.ini: document the dsnoop setup and shared-device pattern.
README, CLAUDE.md: document the full setup procedure.

docker-compose.yml

@@ -4,9 +4,11 @@ services:
     volumes:
       - ./config.ini:/app/config.ini:ro
       - ./recordings:/app/recordings   # matches output_directory = recordings in config.ini
+      - ./asound.conf:/etc/asound.conf:ro  # dsnoop shared-capture config
     # Soundcard (ALSA) access — comment out if you only record streams
     devices:
       - /dev/snd:/dev/snd
+    ipc: host                          # share IPC namespace with host so dsnoop shared memory works
     restart: unless-stopped
     stop_grace_period: 30s             # allow time for open files to be closed cleanly