Completes local-dev-master-slave-setup: dual-instance frontend tooling, module-capability gating, and master/slave protocol self-healing fixes

Frontend (Unit 2 completion): dual dev-server tooling (pnpm dev:slave,
pnpm dev:all), per-instance browser tab titles, and a backend
capability check (SystemController + useSystemCapabilities +
ModuleGuard) so a Master-only page is hidden on a slave instance
instead of assuming every backend has every module.

Master/slave protocol fixes surfaced by actually running master and
slave side by side locally:
- Deactivating a CMS instance (Inactive) now releases the slave's
  master gate instead of leaving it stuck on its last pushed status.
- The periodic integrity check now also re-pushes status to every
  reachable slave (previously URL-verification only) and runs once
  immediately on startup.
- Added the originally-specified (but never implemented) slave-pull
  path: a slave now periodically polls its own status from the master
  (GET /api/v1/SlaveStatus) and fails open to Available if the master
  is unreachable for too long, complementing the existing push.
- The slave's own Settings page can no longer "successfully" change
  local availability while the master controls it; it's now locked
  with an explanatory banner and the backend rejects the write with
  409 instead of silently no-op'ing it.
- CMS instance status badges now match the dashboard's color/icon
  styling instead of a plain grey badge.

Also corrected the master-cms-module design docs to match this
as-built behavior, and flagged (without a full rewrite) a larger,
pre-existing divergence between its inception-stage application
design and what construction actually built.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

aidlc-docs/features/master-cms-module/construction/master-backend/code/code-summary.md

@@ -61,3 +61,14 @@ This generates the `Migrations/` folder contents. The migration is applied autom
 - `Microsoft.Extensions.Http.Resilience` version `9.6.0` — verify/update during `dotnet restore` if a newer version is available for .NET 10
 - `IntegrityCheckIntervalMinutes = 0` in tests forces immediate PeriodicTimer ticks (valid for test scenarios only)
 - Slave-side endpoints (`/api/v1/master/register`, `/api/v1/master/status`, `/api/v1/master/registered-url`) are implemented in Unit 2 (slave-availability-extension)
+
+## Addendum — 2026-07-04 (added outside this unit's original scope, in `local-dev-master-slave-setup` follow-up fixes)
+
+This unit predates the following; see `slave-availability-extension/functional-design/business-rules.md` Rule Set 5 and `application-design/application-design.md` for the full picture:
+
+| File | Description |
+|------|-------------|
+| `Controllers/SlaveStatusController.cs` | New. `[AllowAnonymous]` `GET /api/v1/SlaveStatus`; authenticates via `X-Master-Api-Key` header matched against each active `CmsInstance`'s decrypted key; lets a slave pull its own status instead of relying solely on the master's push |
+| `Services/ICmsInstanceService.cs` / `CmsInstanceService.cs` | `GetStatusForApiKeyAsync(plainApiKey)` added; `UpdateStatusAsync`'s `Inactive` branch now pushes `Available`/null to release the gate (previously a no-op); `VerifyIntegrityAsync` now also re-pushes persisted status to every reachable active slave each cycle |
+| `BackgroundServices/IntegrityCheckBackgroundService.cs` | Now runs one tick immediately on startup, in addition to the periodic timer |
+| `Models/SlaveStatusPollResponse` (in `ICmsInstanceService.cs`) | New record: `IsAvailable`, `DisableMessage` |