feat: move rss to it's own module and add atom support

remote/rss-service/README.md

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+# RSS Service
+
+Serves public feed endpoints for Noisebell.
+
+This service does not store history. On each request it fetches the current state from `cache-service` and renders that one current state as RSS or Atom.
+
+## States
+
+- `open`: Noisebridge is open right now.
+- `closed`: Noisebridge is closed right now.
+- `offline`: the cache cannot currently confirm the Pi state.
+
+## Feed behavior
+
+- Feeds are current-state only, not historical archives.
+- Each feed emits either one item or zero items.
+- The item `guid` changes when the current status event changes, so feed readers can treat it as new.
+- Filtered feeds omit the item when the current state does not match that feed.
+
+## Endpoints
+
+| Format | Purpose | Path |
+|--------|---------|------|
+| JSON | Current status passthrough | `/status` |
+| RSS | All states | `/all/rss.xml` |
+| Atom | All states | `/all/atom.xml` |
+| RSS | Open + closed only | `/door/rss.xml` |
+| Atom | Open + closed only | `/door/atom.xml` |
+| RSS | Open only | `/open/rss.xml` |
+| Atom | Open only | `/open/atom.xml` |
+| HTTP | Health check | `/health` |
+| HTTP | Redirect to docs | `/` |
+
+Current aliases:
+
+- `/rss.xml` currently serves the same feed as `/all/rss.xml`.
+- `/atom.xml` currently serves the same feed as `/all/atom.xml`.
+- `/` redirects to the repo README for this module.
+
+## Feed text
+
+Items use custom titles:
+
+- `Noisebridge opened`
+- `Noisebridge closed`
+- `Noisebridge sensor went offline`
+
+Descriptions include:
+
+- the current state
+- when that state began, if known
+- when the cache last confirmed the state
+
+## Caching and polling hints
+
+- RSS includes `<ttl>1`, which suggests a 1-minute polling interval.
+- HTTP responses include `Cache-Control: public, max-age=60`.
+- HTTP responses also include `ETag` and `Last-Modified` so clients can revalidate cheaply.
+
+## Atom vs RSS
+
+RSS and Atom are both XML feed formats.
+
+- RSS is older and very widely supported.
+- Atom is newer and has a more regular structure.
+- For this service they contain the same information in two different standard formats.
+
+## Configuration
+
+| Variable | Default | Description |
+|----------|---------|-------------|
+| `NOISEBELL_RSS_CACHE_URL` | required | Base URL of `cache-service` |
+| `NOISEBELL_RSS_PORT` | `3002` | Listen port |
+| `NOISEBELL_RSS_HTTP_TIMEOUT_SECS` | `10` | Timeout when requesting cache status |