Module system: discovery, route mounting, client loader, clock module

Milestone 2. Server scans modules/*/module.json, serves module assets at
/modules/<id>/, mounts optional server routes (exact + trailing-/* prefix
patterns) under /api/modules/<id>/ with per-module config from
config/modules.json, and lists enabled manifests at GET /api/modules.
Shell dynamically imports each module's client.js and hands it the
moduleApi (config, instanceSettings, scoped fetch, auto-reconnecting sse,
setStatus). Clock module proves the contract end to end; password-typed
config fields never leave the server.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RNYUv9EyrGSFmCnmH7WYKH

modules/clock/module.json

@@ -0,0 +1,23 @@
+{
+  "id": "clock",
+  "name": "Clock",
+  "version": "1.0.0",
+  "description": "Big booth clock — the smallest possible ProdDash module",
+  "client": "client.js",
+  "style": "style.css",
+  "minSize": { "w": 2, "h": 1 },
+  "defaultSize": { "w": 3, "h": 2 },
+  "instanceSchema": {
+    "format": {
+      "type": "select",
+      "label": "Format",
+      "default": "24h",
+      "options": [
+        { "value": "24h", "label": "24-hour" },
+        { "value": "12h", "label": "12-hour" }
+      ]
+    },
+    "showSeconds": { "type": "boolean", "label": "Show seconds", "default": true },
+    "showDate": { "type": "boolean", "label": "Show date", "default": false }
+  }
+}