feat(server): add Fleet dashboard surface with mock data (#547)

[EnmaJim]

Linked issue

Closes #547

Summary of changes

First pass of the Fleet dashboard in the Mothership (src/server-dotnet), built against
mock JSON fixtures so the UI is reviewable now. Component structure and API call
signatures are finalized here so the live wire-up (#598) is a straight mock→real swap.

Per the #551 coordination, this issue's tab-restructure step is dropped (the v4 shell/rail
shipped in #624); #547 adds the Fleet surface as content that slots into the existing rail.

• Fleet is the landing page (/) — the Q&A dashboard is re-homed to /decisions (the
  Decisions rail slot); a Fleet rail item is added and the active item is driven by
  ViewData["ActiveRail"].
• Instance health cards — status LED, uptime, active workflow, task count; drone
  utilization (load/max, success rate, avg duration) for drone nodes.
• Stale-detection UI — stale instances get a distinct badge/colour.
• Alert banner + notification rail — active alerts with severity + timestamp; a separate
  alert history panel for cleared alerts with resolved timestamps.
• Swappable data seam — IFleetService with a MockFleetService reading
  Mock/fleet-instances.json / fleet-alerts.json; endpoints GET /api/fleet/instances and
  GET /api/fleet/alerts?status=. Fleet dashboard live wire-up — instance data, alerts and auth #598 swaps only the service impl (config-selected, mirroring
  the existing attachment-storage backend swap).
• Auth — DashboardAuthMiddleware extended to gate /decisions and /api/fleet/* exactly
  like the existing dashboard routes.

Screenshots / recordings

Fleet dashboard — the new landing page (/), mock data: alert banner, fleet stats, instance
health cards (outposts + drones), drone utilization, stale badges, alert rail, and alert history.

Q&A dashboard re-homed to /decisions (Decisions rail item now active; Fleet took the root):

Testing notes

• Build + unit/integration (the server.yml gate):

  dotnet build src/server-dotnet/dotbot.sln --configuration Release
  dotnet test  src/server-dotnet/dotbot.sln --configuration Release
  

  Result: 364 passed / 2 pre-existing skips / 0 warnings. New tests:
  • Unit/MockFleetServiceTests.cs — fixture parsing, drone-vs-outpost utilization, active/cleared
    alert filtering, fail-fast on a missing fixture.
  • Integration/FleetApiTests.cs — GET /api/fleet/* end-to-end through the auth pipeline +
    real fixtures.
  • Integration/FleetPageTests.cs — / renders Fleet, /decisions renders the Q&A dashboard,
    rail active-state correct.

Checklist

• Tests added or updated — unit + integration + page-render (Mothership dashboard had none before)
• Docs updated (if behaviour changed) — n/a, mock UI scaffolding; no documented behaviour changed
• Linked issue exists — Closes Fleet dashboard: UI scaffolding with mock data #547
• Follows the contribution guide