Add Nginx web server for browsing recordings

Serves the recordings directory over HTTP with autoindex on port 8080.
Users can browse by date and play MP3 files natively in the browser.

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

openspec/changes/archive/2026-03-19-web-hosting-recordings/proposal.md

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+## Why
+
+Recordings from the RTLSDR-Airband CB scanner are stored as MP3 files on the Pi's filesystem (`/home/m/recordings/`), organized by channel and date. There's currently no way to browse or listen to them without SSH access. A web server with directory listing would make recordings browsable and playable from any device on the local network.
+
+## What Changes
+
+- Add an Nginx container to `docker-compose.yml` that serves the recordings directory with autoindex enabled
+- Users browse directories and click MP3 files to play them natively in the browser
+
+## Capabilities
+
+### New Capabilities
+- `recording-browser`: Nginx autoindex serving the recordings directory over HTTP, with browser-native MP3 playback
+
+### Modified Capabilities
+
+_(none)_
+
+## Impact
+
+- **Docker deployment**: New Nginx service added to `docker-compose.yml`, sharing the `/home/m/recordings` volume (read-only)
+- **Pi resources**: Negligible — Nginx Alpine uses ~2MB RAM
+- **Network**: Exposes HTTP port on the Pi's local network