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>

api/models/__init__.py

@@ -5,6 +5,8 @@ from api.models.workstream_dependency import WorkstreamDependency
 from api.models.task import Task, TaskStatus, TaskPriority
 from api.models.decision import Decision, DecisionType, DecisionStatus
 from api.models.progress_event import ProgressEvent
+from api.models.extension_point import ExtensionPoint, EPStatus
+from api.models.technical_debt import TechnicalDebt, TDStatus
 
 __all__ = [
     "Base",
@@ -14,4 +16,6 @@ __all__ = [
     "Task", "TaskStatus", "TaskPriority",
     "Decision", "DecisionType", "DecisionStatus",
     "ProgressEvent",
+    "ExtensionPoint", "EPStatus",
+    "TechnicalDebt", "TDStatus",
 ]