Upgrade ntfy v2.17.0 → v2.19.2 (#305)

## Summary
- Upgrade ntfy from v2.17.0 to v2.19.2
- Update Dockerfile and Nix build definitions with new version, commit SHA, and hashes
- Add `subPackages = [ "." ]` to Nix build to handle new `tools/loadtest` module in upstream

## Upstream changes (no breaking changes)
- **v2.18.0:** Experimental PostgreSQL backend support
- **v2.19.0:** PostgreSQL read replica support, notification sound throttling
- **v2.19.1-2:** PostgreSQL bug fixes, web push race condition fix

## Test plan
- [ ] Container builds complete on Forgejo Actions (both Dockerfile and Nix)
- [ ] Update kustomization.yaml `newTag` to the built nix image tag
- [ ] `argocd app set ntfy --revision upgrade/ntfy-v2.19.2 && argocd app sync ntfy`
- [ ] Verify ntfy health: `curl https://ntfy.ops.eblu.me/v1/health`
- [ ] Send a test notification

Reviewed-on: #305

argocd/manifests/ntfy/kustomization.yaml

@@ -8,7 +8,7 @@ resources:
   - ingress-tailscale.yaml
 images:
   - name: registry.ops.eblu.me/blumeops/ntfy
-    newTag: v2.17.0-613f05d-nix
+    newTag: v2.19.2-f654c57-nix
 configMapGenerator:
   - name: ntfy-config
     files: