Serve static maintenance page with 503 when PocketBase is unreachable

docs/deployment.md

@@ -128,6 +128,31 @@ development and the tests and are **not** used on Unraid — see `entwicklung.md
 
 ---
 
+## Outage: PocketBase unreachable
+
+If `eb-pocketbase` is down while `eb-blazor` runs, the app does not render
+half-empty pages (the menu comes from PocketBase too). A gate in `Program.cs`
+answers every **page** request with the static `wwwroot/maintenance.html`:
+
+| Request | Answer while PocketBase is down |
+|---|---|
+| Any page (`/`, `/board`, `/posts/…`, unknown slugs) | **503**, `Retry-After: 60`, maintenance page |
+| `/health` | **503** `PocketBase unreachable` (for Uptime Kuma) |
+| `/events.ics` | 200 with an empty calendar, so subscriptions keep working |
+| `/events/{id}.ics` | 503 |
+| Static files (CSS, fonts, PDFs, `/maintenance.html`) | served as usual |
+
+The gate caches the health check for **10 seconds** (`PocketBaseHealthCache`). So
+after PocketBase stops, pages can render for up to 10 s more (the components
+degrade on their own in that window: empty menu, a short note, 503), and after it
+starts again the maintenance page can stay up for up to 10 s. No restart of the
+web app is needed — the site comes back by itself.
+
+`maintenance.html` carries its styles inline, as a copy of the tokens from
+`app.css`. When colours change there, copy them over.
+
+---
+
 ## Rollback
 
 `scripts/release.sh` additionally tags each release with the short commit SHA,