feat(state-hub): add Extension Points and Technical Debt tracking

New entity types (DB tables, API routers, Pydantic schemas, Alembic
migration a3f1c2d4e5b6):
- extension_points: ep_id, domain, title, ep_type, status, priority,
  location, description, topic_id, workstream_id
- technical_debt: td_id, domain, title, debt_type, severity, status,
  location, description, topic_id, workstream_id

MCP server: 6 new tools — register_extension_point, list_extension_points,
update_ep_status, register_technical_debt, list_technical_debt,
update_td_status (each write emits a progress_event)

Dashboard: two new pages (extensions.md, techdept.md) with KPI sidebar,
charts, urgent-items section, and filterable card lists. Both added to
nav in observablehq.config.js.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

dashboard/observablehq.config.js

@@ -7,6 +7,8 @@ export default {
     { name: "Tasks", path: "/tasks" },
     { name: "Decisions", path: "/decisions" },
     { name: "Progress", path: "/progress" },
+    { name: "Extension Points", path: "/extensions" },
+    { name: "Technical Debt", path: "/techdept" },
     {
       name: "Reference",
       pages: [