Add recording cleanup container to remove noise files

Alpine-based sdr-cleanup container runs a find loop every 5 minutes,
deleting MP3 files under 3 KB (configurable via SIZE_THRESHOLD env var)
to keep the web browser and Samba share free of false-positive recordings.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

openspec/changes/archive/2026-03-21-recording-cleanup-cron/design.md

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+## Context
+
+The SDR recorder running in Docker on `sdr-pi` writes MP3 files to `/home/m/recordings/` (mounted from the host). Despite squelch tuning, many files are noise — today's data shows 91 files totaling 476 KB, with the majority under 2 KB. These clutter the Nginx web browser and Samba share, making it hard to find real transmissions.
+
+The recorder container runs as root (required for USB/SDR hardware access) and writes files owned by root, so cleanup must also run as root.
+
+## Goals / Non-Goals
+
+**Goals:**
+- Automatically remove recordings that are too small to contain real audio
+- Make the size threshold easily adjustable via environment variable
+- Keep everything version-controlled in docker-compose
+- Run frequently enough that the web/Samba views stay clean
+
+**Non-Goals:**
+- Duration-based or content-based analysis (too complex, needs extra tools)
+- Age-based retention policy (handled manually for now)
+
+## Decisions
+
+### Dedicated cleanup container in docker-compose
+Add a lightweight Alpine container (`sdr-cleanup`) to docker-compose that shares the `/home/m/recordings` volume. This follows Docker's separation-of-concerns model — one container per responsibility. The container runs a simple `find -delete` loop with `sleep` instead of requiring a cron daemon. Since it runs as root (Alpine default), it can delete the root-owned files written by the recorder.
+
+Alternatives considered:
+- **Host crontab**: Simplest, but files are root-owned so needs `sudo crontab`, and isn't version-controlled in docker-compose
+- **Cron inside recorder container**: Not our image, would need custom build
+- **Systemd timer on host**: More ceremony than cron for the same result
+
+### File size threshold as the filter heuristic
+Use file size (default 3 KB) rather than MP3 duration. This avoids needing `ffprobe` or similar tools. From today's data, files under 3 KB are consistently noise — real transmissions start around 5 KB. Configurable via `SIZE_THRESHOLD` environment variable in docker-compose.
+
+### 5-minute sleep loop
+Run every 5 minutes via `sleep 300` in a loop. Lightweight and keeps the browsing experience clean.
+
+### Only delete files older than 1 minute
+Add a minimum age filter (`-mmin +1`) to avoid deleting files still being written by rtl_airband.
+
+## Risks / Trade-offs
+
+- **Threshold too aggressive** → Real short transmissions deleted. Mitigation: start conservative at 3 KB, tune via environment variable.
+- **Race condition with active writes** → The `-mmin +1` guard ensures we never delete a file rtl_airband is still writing.
+- **Container restart** → If the cleanup container crashes, `restart: unless-stopped` brings it back. No state to lose.