feat(milestone): merge phase/02 cohort-dashboard → milestone/v0.4-operator-tier

Phase 2 complete — Cohort Dashboard + Aggregation:
- Cohort aggregation pipeline (k-anon ≥10 write-time suppression, async hook, nightly 03:00 CT reconcile)
- 4 auth-gated operator API endpoints (cohort, mastery, failure-patterns, credentials)
- React cohort dashboard (BrowserRouter, login, 3 views, inline SVG sparklines, auth gate)
- SPA fallback via SpaStaticFiles subclass (G-041 — NOT catch-all route)
- G-038 differencing-attack test (unit + API e2e)
- 317 pytest pass, 36 skip, 0 fail; 17/17 vitest pass; npm build + typecheck clean

---ci---
project: praxis
phase: 2
milestone: v0.4
status: complete
requirements:
  covered: [REQ-DASH-01, REQ-NFR-DASH-01, REQ-NFR-DASH-02, REQ-MT-02]
  partial: []
---/ci---

server/operator/failure_patterns.py

@@ -0,0 +1,44 @@
+"""GET /api/operator/failure-patterns — failure patterns view (TASK-08-03, D-053).
+
+Auth-gated. Returns failure pattern metrics: failure_mode frequency (cells
+with metric prefix `failure_mode:`) + branch outcome distribution (cells
+with metric prefix `branch:`). Weak-spot rubric criteria (mean < 3.0) are
+highlighted by the frontend. All k-anonymized.
+"""
+
+from __future__ import annotations
+
+import datetime as _dt
+
+from fastapi import APIRouter, Depends, Request
+
+from server.auth.dependencies import current_operator
+from server.auth.models import Operator
+from server.operator._common import (
+    ViewResponse,
+    all_recent_aggregates,
+    group_by_path,
+    require_pg_store,
+)
+
+router = APIRouter(prefix="/api/operator", tags=["operator-failure-patterns"])
+
+
+def _is_failure_metric(metric: str) -> bool:
+    return metric.startswith("failure_mode:") or metric.startswith("branch:")
+
+
+@router.get("/failure-patterns", response_model=ViewResponse)
+async def failure_patterns_view(
+    request: Request,
+    op: Operator = Depends(current_operator),
+) -> ViewResponse:
+    pg_store = await require_pg_store(request)
+    since = _dt.date.today() - _dt.timedelta(days=30)
+    rows = await all_recent_aggregates(pg_store, since)
+    failure_rows = [r for r in rows if _is_failure_metric(r.get("metric", ""))]
+    views, last_updated = group_by_path(failure_rows)
+    return ViewResponse(views=views, last_updated=last_updated)
+
+
+__all__ = ["router"]