Rename maglev-frontend → maglevd-frontend; v0.9.1; API RX/TX pulse

Rename the web dashboard binary to maglevd-frontend and move it to
/usr/sbin (it's a daemon and belongs with maglevd). The systemd unit
name stays vpp-maglev-frontend.service since that prefix is the
package name. Manpage, README, user-guide, and debian packaging all
updated in lockstep; bump to 0.9.1 for the first real release.

All frontend env vars are now prefixed MAGLEV_FRONTEND_ so a single
/etc/default/vpp-maglev can be shared with maglevd without collisions.
Every flag has an env equivalent for Docker use. MAGLEV_FRONTEND_USER
and MAGLEV_FRONTEND_PASSWORD still gate the /admin surface.

VPPInfoPanel now pulses "API: ↑↓" indicators in the zippy title
whenever a vpp-api-send / vpp-api-recv log event arrives on the SSE
stream for the scoped maglevd — 250ms blue flash, re-triggerable,
with the two arrows tightly kerned via negative letter-spacing.

docs/user-guide.md

@@ -241,3 +241,83 @@ keyword. The `?` character is not added to the input line.
 
 Commands and keywords support **prefix matching**: typing `sh ba` is equivalent
 to `show backends`, and `sh ba nginx0` is equivalent to `show backends nginx0`.
+
+---
+
+## maglevd-frontend
+
+`maglevd-frontend` is an optional web dashboard that connects to one or
+more running `maglevd` instances over gRPC and renders a live view of
+frontends, backends, health checks, and VPP load-balancer state. It is
+a single Go binary with the SolidJS SPA embedded via `//go:embed`; no
+runtime file dependencies.
+
+Installed by the Debian package to `/usr/sbin/maglevd-frontend` but
+**not** enabled by default — the operator opts in via:
+
+```sh
+systemctl enable --now vpp-maglev-frontend
+```
+
+The systemd unit (`vpp-maglev-frontend.service`) reads its arguments
+from `/etc/default/vpp-maglev` via `MAGLEV_FRONTEND_ARGS`. The same
+env file is shared with `maglevd`; all `maglevd-frontend`-specific
+variables are prefixed with `MAGLEV_FRONTEND_` so there's no overlap.
+
+### Flags
+
+| Flag | Environment variable | Default | Description |
+|---|---|---|---|
+| `--server` | `MAGLEV_FRONTEND_SERVERS` | *(required)* | Comma-separated list of `host:port` maglevd addresses. |
+| `--listen` | `MAGLEV_FRONTEND_LISTEN` | `:8080` | HTTP bind address. |
+| `--log-level` | `MAGLEV_FRONTEND_LOG_LEVEL` | `info` | Structured-log verbosity for `maglevd-frontend`'s own logs. |
+| `--version` | — | — | Print version, commit hash, and build date, then exit. |
+
+In addition to flags, two env-only variables control the admin surface:
+
+| Environment variable | Purpose |
+|---|---|
+| `MAGLEV_FRONTEND_USER` | HTTP basic-auth username for `/admin/`. |
+| `MAGLEV_FRONTEND_PASSWORD` | HTTP basic-auth password for `/admin/`. |
+
+When **both** are set and non-empty the admin surface is mounted and
+the SPA's "admin…" toggle becomes visible. When either is missing or
+empty the `/admin/` route returns 404 and the SPA hides the toggle —
+`/view/` is always reachable read-only.
+
+### HTTP surface
+
+- **`/view/`** — static SPA (dashboard). No authentication.
+- **`/view/api/state`**, **`/view/api/state/{name}`** — full JSON
+  snapshot for every maglevd, or one maglevd.
+- **`/view/api/maglevds`** — configured maglevds and connection status.
+- **`/view/api/version`** — build info + `admin_enabled` flag.
+- **`/view/api/events`** — Server-Sent Events stream; log, backend,
+  frontend, maglevd-status, vpp-status events with
+  `Last-Event-ID` replay from a 30-second / 2000-event ring buffer.
+- **`/healthz`** — liveness; returns 200 if the HTTP server is up.
+- **`/admin/`** — SPA shell behind basic auth (when configured).
+- **`POST /admin/api/{maglevd}/backend/{name}/{action}`** — backend
+  lifecycle action. `action` is `pause`, `resume`, `enable`, or
+  `disable`. Returns the fresh backend snapshot as JSON.
+- **`POST /admin/api/{maglevd}/frontend/{fe}/pool/{pool}/backend/{name}/weight`**
+  — weight change. Body: `{"weight": 0-100, "flush": bool}`. When
+  `flush=true`, VPP's flow table for the backend is cleared;
+  otherwise only the new-buckets map is updated and existing
+  sessions keep reaching the backend until they finish.
+
+### Reverse-proxy requirements (SSE)
+
+Nginx, HAProxy, or any proxy in front of `maglevd-frontend` must:
+
+- Disable buffering on the events endpoint. `X-Accel-Buffering: no`
+  is sent by the server; a global `proxy_buffering off;` in the
+  nginx server block is the more robust answer.
+- Raise `proxy_read_timeout` to at least 300s so the stream isn't
+  torn down between the 15-second `: ping` heartbeats the server
+  sends.
+- Not wrap the events endpoint in any gzip/brotli middleware —
+  response compression buffers until its window fills and destroys
+  the live-stream property.
+
+See `maglevd-frontend(8)` for the full reference.