feat(modules): mount the modules directory as a volume (phase 2, PR 9)

Closes Phase 2. Modules live on a mount, never in the image — that is what
lets an operator add one to a pull-only deployment without building anything.

`./modules` is a bind mount rather than a named volume: placing a module
directory by hand is a supported install (MODULE_SYSTEM.md §2.5), and that has
to be doable from the host rather than through `docker cp`. Read-write, because
the admin panel's install/uninstall unpacks and removes directories there.

The directory is tracked via its README so it exists in the checkout with the
operator's own ownership — Docker recreates a missing bind-mount source as
root:root, which the container user could not then write. `.dockerignore`
excludes it so a module in the builder's working tree can never ship inside an
image.

Also corrects the route-manifest generator's list of filesystem-conditional
mounts, which never picked up `/modules` when PR 7 added it. Comment only; the
generator filters on an allowlist, so its behaviour was already right.

Verified against a real container, not just a parsed compose file: image
carries an empty node-owned /app/modules despite a module in the build context;
a module on the bind mount loads, mounts, replays and reaches `started`;
`/api/v1/public/modules` lists it; the chunk serves from the entry's directory
only (server source and module.json 404) with `no-cache`; the injected tag
follows core's bundle; and in Chrome the page renders on first paint inside
core's PublicLayout with its nav row interleaved into core's public nav, under
enforced `script-src 'self'` with zero CSP reports and no console errors.
Removing the directory by hand reconciles the row to `startup_failed`/`require`
and leaves core healthy with no injection.

933 server + 160 client tests pass, manifest unchanged at 230 routes, swagger
regenerates byte-identical.

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

server/scripts/routeManifest.js

@@ -16,10 +16,11 @@
  * derived (only annotated routes appear) and documents intent; this records reality.
  *
  * Scope: only `/api/**` and `/.well-known/**` from the public app, plus everything
- * on the internal app. Three mounts in app.js are *filesystem* conditional — the SPA
- * catch-all `GET *`, the `/brand` static mount and swagger-ui's `/api/docs` static
- * assets — so including them would make the output depend on whether CI had built
- * the client. Static mounts are not API contract.
+ * on the internal app. Four mounts in app.js are *filesystem* conditional — the SPA
+ * catch-all `GET *`, the `/brand` static mount, installed modules' `/modules/<id>`
+ * chunks and swagger-ui's `/api/docs` static assets — so including them would make
+ * the output depend on whether CI had built the client, or on which modules were
+ * mounted. Static mounts are not API contract.
  *
  * Usage:
  *   npm run routes:manifest            # write server/routes.manifest.json (+ guards)
@@ -56,7 +57,8 @@ const GUARDS_COMMENT =
   '`npm run routes:manifest`.'
 
 // Only these prefixes are contract. Everything else the public app serves (SPA
-// shell, /uploads, /brand, swagger-ui assets) is static delivery, not API surface.
+// shell, /uploads, /brand, /modules, swagger-ui assets) is static delivery, not
+// API surface.
 const PUBLIC_PREFIXES = ['/api/', '/.well-known/']
 
 /**