feat(home-assistant): set internal/external URLs for mobile_app push

The HA Companion app needs an external_url for clickable notification
deep-links to resolve when the phone is off the home Wi-Fi. Reach is via
Tailscale (Headscale tailnet solar.internal), so external_url points at
the FQDN jupiter.solar.internal:8123; internal_url stays on the bare
hostname for LAN-attached devices.

Phase A only: device registration + notify group + smoke-test land in a
follow-up commit once Companion has registered real mobile_app_<slug>
service names. See docs/superpowers/specs/2026-05-18-ha-push-notifications.md.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/superpowers/specs/2026-05-18-ha-push-notifications.md

@@ -0,0 +1,111 @@
+# Smartphone push notifications from Home Assistant
+
+## Goal
+
+Send push notifications from jupiter's Home Assistant to the user's
+smartphones over the home Tailscale (Headscale) tailnet `solar.internal`.
+
+## Architecture
+
+- HA's `mobile_app` integration is enabled (already in `extraComponents`
+  and present as `mobile_app = {}` in config).
+- Each smartphone runs the **HA Companion app**, signs in to HA, and
+  auto-registers as a `notify.mobile_app_<device_slug>` service.
+- Reach: phones connect to HA via Tailscale, so HA's `external_url` is
+  set to the Headscale FQDN `http://jupiter.solar.internal:8123`. The
+  `internal_url` is `http://jupiter:8123` for LAN-attached devices.
+- No public exposure, no reverse proxy, no TLS termination in scope.
+
+## Phase A — done in this branch
+
+NixOS module change in `modules/environments/home-assistant/default.nix`:
+
+```nix
+homeassistant = {
+  name = "Home - Rechberg";
+  unit_system = "metric";
+  internal_url = "http://${hostName}:8123";
+  external_url = "http://jupiter.solar.internal:8123";
+};
+```
+
+After deploy, perform the user-side registration:
+
+1. Install the Companion app:
+   - iOS: search "Home Assistant" in the App Store.
+   - Android: search "Home Assistant" in Google Play.
+2. Ensure Tailscale is running and connected on the phone.
+3. Open the Companion app. When asked to connect, enter
+   `http://jupiter.solar.internal:8123` and sign in with the HA account.
+4. Approve the registration prompt in HA.
+5. In HA, go to **Settings → Devices & Services → Mobile App** and
+   confirm the phone appears as a device.
+6. In HA, go to **Developer Tools → Services**, type `notify.mobile_app_`
+   and note the exact service slug for each phone (e.g.
+   `notify.mobile_app_iphone_finn`). These slugs are needed for Phase B.
+
+### Verifying Phase A end-to-end
+
+Build-time:
+
+```
+nix eval '.#nixosConfigurations.jupiter.config.services.home-assistant.config.homeassistant' \
+  --extra-experimental-features 'nix-command flakes'
+```
+
+Expect the rendered attrset to contain both `external_url` and
+`internal_url`.
+
+Deploy on jupiter:
+
+```
+sudo nixos-rebuild switch --flake '.#jupiter'
+systemctl status home-assistant
+journalctl -u home-assistant -n 50 --no-pager
+```
+
+Functional check after Companion sign-in:
+
+- HA UI → **Developer Tools → Services** → choose
+  `notify.mobile_app_<your_device>` → service data
+  `{ "message": "Phase A test" }` → **Call Service** → push arrives on
+  the phone.
+
+## Phase B — follow-up commit (after registration)
+
+Once device slugs are known, a separate commit adds:
+
+1. A `notify` group fanning out to every registered phone:
+
+   ```nix
+   notify = [
+     {
+       name = "all_phones";
+       platform = "group";
+       services = [
+         { service = "mobile_app_<slug_1>"; }
+         { service = "mobile_app_<slug_2>"; }
+       ];
+     }
+   ];
+   ```
+
+2. A smoke-test mechanism. Approach to be decided in Phase B based on
+   whether future Nix-managed automations are expected:
+
+   - Pragmatic: document a one-time UI call to `notify.all_phones` from
+     Developer Tools (no automation in YAML).
+   - Compromise: switch `automation = "!include automations.yaml"` to
+     `!include_dir_merge_list automations/` so a Nix-managed
+     `00-smoke-test.yaml` can coexist with UI-editable automations.
+
+### Verifying Phase B end-to-end
+
+- Restart HA, watch `journalctl -u home-assistant` for YAML schema errors.
+- Call `notify.all_phones` from Developer Tools — every registered phone
+  receives the push.
+
+## Open items
+
+- After Companion registration, collect the `mobile_app_<slug>` service
+  names from HA and update this spec + open Phase B PR.

modules/environments/home-assistant/default.nix

@@ -40,6 +40,8 @@ in
       homeassistant = {
         name = "Home - Rechberg";
         unit_system = "metric";
+        internal_url = "http://${hostName}:8123";
+        external_url = "http://jupiter.solar.internal:8123";
       };
       mobile_app = {};
       automation = "!include automations.yaml";