feat(modules): publish the installed-module list at /api/v1/public/modules

Phase 2, PR 6 of docs/website/MODULE_SYSTEM.md 2.7 — the first module-system
URL a client can see. The SPA and the Android app feature-detect against the
capabilities a module declares; the shape is settled in MODULE_API.md 2.9.

Four decisions, and what is absent from the payload is most of the design:

* started modules only. A module that is disabled or failed to load is
  ABSENT, exactly as 4.4 already leaves its routes and its nav absent, so a
  client renders a site without that capability rather than advertising one
  that 503s.
* no state, failure_stage or failure_reason. Where a module broke belongs to
  the admin Modules screen, and the reason is an exception string from inside
  core — not anonymous-visitor business.
* no client chunk URL. htmlShell injects a script tag per started module
  (3.1.3), so the browser is handed the tag rather than a URL to fetch. This
  endpoint feature-detects; it does not load. MODULE_SYSTEM 2.6 step 4 is
  amended to match (API 6.7).
* no siteMode gate and no database — the same class as /public/status and
  /public/version, so a client can still feature-detect during maintenance.

It is a capability router of its own rather than a fifth singleton in
site.router.js, and that is load-bearing: the loader's prefix-collision probe
reads the live tier stack and skips root-mounted layers, because a use('/', ...)
matches every path. A route inside the root-mounted site router would be
invisible to it — mounting use('/modules', ...) is what makes "no module may
claim /modules" a rule the loader enforces.

910 tests pass (+9, every one on the boundary — what must NOT appear).
routes.manifest.json gains exactly the one route and routes.guards.json records
it with an empty gates list, which is itself the assertion that it is ungated.

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

server/routes.manifest.json

@@ -781,6 +781,10 @@
       "method": "POST",
       "path": "/api/v1/public/contact"
     },
+    {
+      "method": "GET",
+      "path": "/api/v1/public/modules"
+    },
     {
       "method": "GET",
       "path": "/api/v1/public/pages/:id/preview/:token"