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/specs/recording-cleanup/spec.md

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+### Requirement: Cleanup script removes small recordings
+The system SHALL provide a shell script that deletes MP3 files below a configurable size threshold from the recordings directory. The default threshold SHALL be 3 KB (3072 bytes).
+
+#### Scenario: File below threshold is deleted
+- **WHEN** a `.mp3` file in `/home/m/recordings/` is smaller than the configured size threshold and older than 1 minute
+- **THEN** the file is deleted
+
+#### Scenario: File above threshold is preserved
+- **WHEN** a `.mp3` file in `/home/m/recordings/` is equal to or larger than the configured size threshold
+- **THEN** the file is not deleted
+
+#### Scenario: File currently being written is preserved
+- **WHEN** a `.mp3` file in `/home/m/recordings/` was modified less than 1 minute ago
+- **THEN** the file is not deleted, regardless of size
+
+### Requirement: Cleanup runs on a schedule
+The system SHALL execute the cleanup script via a cron job on the Pi host, running every 5 minutes.
+
+#### Scenario: Periodic execution
+- **WHEN** 5 minutes have elapsed since the last run
+- **THEN** the cleanup script executes and removes qualifying files
+
+### Requirement: Size threshold is configurable
+The cleanup script SHALL accept the size threshold as a parameter, allowing adjustment without editing the script.
+
+#### Scenario: Custom threshold
+- **WHEN** the script is invoked with a size argument (e.g., `5k`)
+- **THEN** only files below that size are deleted instead of the default 3 KB
+
+### Requirement: Cleanup script is version-controlled
+The cleanup script SHALL be stored in the project repository so that changes are tracked and deployable.
+
+#### Scenario: Script lives in repo
+- **WHEN** a developer checks the repository
+- **THEN** the cleanup script is present at a well-known path with documentation of its purpose