[feat](trx-frontend-http): add Background Decoding Scheduler

Implements a scheduler that retunes the rig automatically when no SSE
clients are connected.  Two modes are supported:

- Grayline: tunes to per-period bookmarks (dawn/day/dusk/night) based on
  an inline NOAA solar algorithm given station lat/lon.
- Time Span: tunes to bookmarks within user-defined UTC windows; midnight-
  spanning intervals supported.

Backend:
- SchedulerStore (PickleDB, sch:{rig_id} keys) in scheduler.rs
- spawn_scheduler_task polls every 30 s, checks context.sse_clients == 0,
  sends SetFreq + SetMode via RigRequest with rig_id_override
- HTTP API: GET/PUT/DELETE /scheduler/{rig_id}, GET …/status
- sse_clients Arc<AtomicUsize> added to FrontendRuntimeContext and shared
  with the SSE counter in build_server (single source of truth)
- /scheduler/ added to Read auth routes (write requires Control)

Frontend:
- Scheduler tab (clock icon, 6th position) with Grayline/TimeSpan UI
- scheduler.js plugin: loads config + bookmarks, live status polling
  every 15 s, write controls hidden for Rx-role users
- CSS .sch-* component styles added to style.css
- SCHEDULER.md design document at repo root

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

SCHEDULER.md

@@ -0,0 +1,119 @@
+# Background Decoding Scheduler
+
+## Overview
+
+The Background Decoding Scheduler automatically retunes the rig to pre-configured
+bookmarks when no users are connected to the HTTP frontend. It runs as a background
+tokio task inside `trx-frontend-http`, polling every 30 seconds.
+
+## Modes
+
+### Disabled (default)
+Scheduler is inactive. Rig is not touched automatically.
+
+### Grayline
+Retunes around the solar terminator (day/night boundary).
+
+The user provides:
+- Station latitude and longitude (decimal degrees)
+- Optional transition window width (minutes, default 20)
+- Bookmark IDs for four periods:
+  - **Dawn** – window around sunrise (`sunrise ± window_min/2`)
+  - **Day** – after dawn until dusk
+  - **Dusk** – window around sunset (`sunset ± window_min/2`)
+  - **Night** – after dusk until next dawn
+
+Period precedence (most specific wins): Dawn > Dusk > Day > Night.
+
+If no bookmark is assigned to a period, the rig is not retuned for that period.
+
+Sunrise/sunset is computed inline using the NOAA simplified algorithm.
+Polar regions (midnight sun / polar night) fall back to Day/Night accordingly.
+
+### TimeSpan
+Retunes according to a list of user-defined time windows (UTC).
+
+Each entry specifies:
+- `start_hhmm` – start of window (e.g. 600 = 06:00 UTC)
+- `end_hhmm` – end of window (e.g. 700 = 07:00 UTC)
+- `bookmark_id` – bookmark to apply
+- `label` – optional human-readable description
+
+Windows that span midnight (`end_hhmm < start_hhmm`) are supported.
+When multiple entries overlap, the first match (by list order) wins.
+
+## Storage
+
+Configuration is stored in PickleDB at `~/.config/trx-rs/scheduler.db`.
+
+Keys: `sch:{rig_id}` → JSON `SchedulerConfig`.
+
+## HTTP API
+
+All read endpoints are accessible at the **Rx** role level.
+Write endpoints require the **Control** role.
+
+| Method | Path | Description |
+|--------|------|-------------|
+| GET | `/scheduler/{rig_id}` | Get scheduler config for a rig |
+| PUT | `/scheduler/{rig_id}` | Save scheduler config (Control only) |
+| DELETE | `/scheduler/{rig_id}` | Reset config to Disabled (Control only) |
+| GET | `/scheduler/{rig_id}/status` | Get last-applied bookmark and next event |
+
+## Activation logic
+
+Every 30 seconds the scheduler task checks:
+1. `context.sse_clients.load() == 0` — no users connected
+2. Active rig has a non-Disabled scheduler config
+3. Current UTC time matches a scheduled window or grayline period
+4. If the matching bookmark differs from `last_applied`, send `SetFreq` + `SetMode`
+
+The scheduler **does not** revert changes when users reconnect. Bookmarks serve as
+a frequency map — the user can retune manually after connecting.
+
+## Data model (Rust)
+
+```rust
+pub enum SchedulerMode { Disabled, Grayline, TimeSpan }
+
+pub struct GraylineConfig {
+    pub lat: f64,
+    pub lon: f64,
+    pub transition_window_min: u32,
+    pub day_bookmark_id: Option<String>,
+    pub night_bookmark_id: Option<String>,
+    pub dawn_bookmark_id: Option<String>,
+    pub dusk_bookmark_id: Option<String>,
+}
+
+pub struct ScheduleEntry {
+    pub id: String,
+    pub start_hhmm: u32,
+    pub end_hhmm: u32,
+    pub bookmark_id: String,
+    pub label: Option<String>,
+}
+
+pub struct SchedulerConfig {
+    pub rig_id: String,
+    pub mode: SchedulerMode,
+    pub grayline: Option<GraylineConfig>,
+    pub entries: Vec<ScheduleEntry>,
+}
+```
+
+## UI (Scheduler tab)
+
+A dedicated sixth tab with a clock icon.
+
+- **Rig selector**: shows active rig (read-only).
+- **Mode picker**: Disabled / Grayline / TimeSpan radio buttons.
+- **Grayline section** (visible when mode = Grayline):
+  - Lat/lon inputs
+  - Transition window slider (5–60 min)
+  - Four bookmark selectors (Dawn / Day / Dusk / Night)
+- **TimeSpan section** (visible when mode = TimeSpan):
+  - Table of entries with Start, End, Bookmark, Label, Remove button
+  - "Add Entry" row at the bottom
+- **Status card**: last applied bookmark name and timestamp.
+- Save button (Control only; form is read-only for Rx users).