feat(widgets): add backend source adapters and per-widget data endpoint

PR 2 of 4 for configurable dashboard widgets.

- Add grafana_url and prometheus_url settings (config.py + compose/env).
- Create WidgetSource protocol and adapters for jellyfin, backups, grafana,
  prometheus, ssh_task, and static sources.
- Add GET /api/widgets/instances/{id}/data endpoint.
- Extract shared dashboard helpers into domain/dashboard.py so widgets and
  the dashboard router reuse the same logic.
- Add adapter and data-endpoint tests.
- Update apply-progress.md.

Verification: ruff clean; backend pytest 200 passed; frontend lint/build green.

backend/src/media_library_viewer_api/domain/dashboard.py

@@ -0,0 +1,91 @@
+"""Dashboard domain helpers shared between routers and widget adapters."""
+
+from __future__ import annotations
+
+import time
+from typing import Any
+
+from media_library_viewer_api.models.backups import BackupDashboardSummary
+from media_library_viewer_api.services.settings_store import SettingsStore
+
+
+def _map_sessions_to_activity_rows(sessions: list[dict[str, Any]]) -> list[dict[str, Any]]:
+    """Normalize Jellyfin sessions into dashboard activity rows."""
+    results: list[dict[str, Any]] = []
+    for session in sessions:
+        item = session.get("NowPlayingItem") or {}
+        play_state = session.get("PlayState") or {}
+        transcoding = session.get("TranscodingInfo") or {}
+
+        has_item = bool(item)
+        series = item.get("SeriesName") or ""
+        title = (
+            (f"{series} - {item.get('Name', '')}" if series else item.get("Name", "Unknown"))
+            if has_item
+            else "(idle)"
+        )
+
+        if not has_item:
+            state_label = "idle"
+        else:
+            state_label = "paused" if play_state.get("IsPaused") else "playing"
+
+        is_transcoding = bool(transcoding)
+        transcode_type: list[str] = []
+        if is_transcoding:
+            if transcoding.get("IsVideoDirect") is False:
+                transcode_type.append("video")
+            if transcoding.get("IsAudioDirect") is False:
+                transcode_type.append("audio")
+            if not transcode_type:
+                transcode_type.append("active")
+
+        results.append(
+            {
+                "user": session.get("UserName") or "Unknown",
+                "title": title,
+                "type": item.get("Type", "") if has_item else "",
+                "state": state_label,
+                "transcoding": "yes" if is_transcoding else "no",
+                "transcoding_type": ", ".join(transcode_type),
+                "device": session.get("DeviceName") or session.get("Client") or "",
+                "session_id": session.get("Id") or "",
+            }
+        )
+    return results
+
+
+def build_backup_dashboard_summary(store: SettingsStore) -> BackupDashboardSummary:
+    """Compute the backup summary shown on the dashboard."""
+    jobs = store.list_backup_jobs()
+    total_jobs = len(jobs)
+
+    cutoff = int(time.time()) - (24 * 60 * 60)
+    recent_runs = []
+    for job in jobs:
+        runs = store.list_backup_runs(job_id=job["id"], limit=1)
+        if runs and runs[0]["started_at"] >= cutoff:
+            recent_runs.append(runs[0])
+
+    successful = sum(1 for r in recent_runs if r["status"] == "success")
+    success_rate = (successful / len(recent_runs) * 100) if recent_runs else 100.0
+
+    alerts = store.list_backup_alerts(acknowledged=False)
+    active_alerts = len(alerts)
+
+    failed_runs = []
+    for job in jobs:
+        runs = store.list_backup_runs(job_id=job["id"], status="failure", limit=1)
+        if runs:
+            failed_runs.append(runs[0])
+
+    last_failed_at = None
+    if failed_runs:
+        last_failed_at = max(r["started_at"] for r in failed_runs)
+
+    return BackupDashboardSummary(
+        total_jobs=total_jobs,
+        success_rate_24h=round(success_rate, 1),
+        active_alerts=active_alerts,
+        last_failed_at=last_failed_at,
+    )