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/schemas/technical_debt.py

@@ -0,0 +1,49 @@
+import uuid
+from datetime import datetime
+
+from pydantic import BaseModel, ConfigDict
+
+from api.models.technical_debt import TDStatus
+
+VALID_SEVERITIES = {"low", "medium", "high", "critical"}
+
+
+class TDCreate(BaseModel):
+    td_id: str | None = None
+    domain: str
+    title: str
+    description: str | None = None
+    location: str | None = None
+    debt_type: str = "other"
+    severity: str = "medium"
+    status: TDStatus = TDStatus.open
+    topic_id: uuid.UUID | None = None
+    workstream_id: uuid.UUID | None = None
+
+
+class TDUpdate(BaseModel):
+    title: str | None = None
+    description: str | None = None
+    location: str | None = None
+    debt_type: str | None = None
+    severity: str | None = None
+    status: TDStatus | None = None
+    workstream_id: uuid.UUID | None = None
+
+
+class TDRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: uuid.UUID
+    td_id: str | None = None
+    domain: str
+    title: str
+    description: str | None = None
+    location: str | None = None
+    debt_type: str
+    severity: str
+    status: TDStatus
+    topic_id: uuid.UUID | None = None
+    workstream_id: uuid.UUID | None = None
+    created_at: datetime
+    updated_at: datetime