feat(P02): complete integration + tech-debt + NFR measurement phase — v0.1.12 tagged

Phase 2 (Integration + Tech-Debt + NFR Measurement) complete.
4 slices, 2 waves, 9 tasks. 4 REQs covered. 60 new tests (469 total).
8 v0.4 P1+ tech-debt findings addressed. Verify: APPROVE_WITH_NOTES.

NFR measurement (p95 latency + guardrail FP/FN), cohort aggregation
assist metrics (5 new metrics, no schema change), assist cost tracking
+ C-3 budget check, tech-debt wave (argon2id offload, cookie-secret
validation, credential enum, f-string SQL, cache persistence, zoneinfo,
audit log, 429 mock).

---ci---
project: praxis
phase: 2
milestone: v0.5
status: complete
requirements:
  covered: [REQ-NFR-ASSIST-01, REQ-IDEATE-04, REQ-IDEATE-06, REQ-IDEATE-07]
  partial: []
---/ci---

server/operator/failure_patterns.py

@@ -1,9 +1,14 @@
-"""GET /api/operator/failure-patterns — failure patterns view (TASK-08-03, D-053).
+"""GET /api/operator/failure-patterns — failure patterns + safety signals (TASK-08-03, TASK-10-02, 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.
+with metric prefix `branch:`) + the assist guardrail block rate safety signal
+(TASK-10-02 — `assist_guardrail_block_rate`). Weak-spot rubric criteria
+(mean < 3.0) are highlighted by the frontend. All k-anonymized.
+
+The `assist_guardrail_block_rate` is a safety signal for operators: a sudden
+spike signals either a prompt regression or learners pushing boundaries. High
+block rate = flag for operator review.
 """
 
 from __future__ import annotations
@@ -23,9 +28,17 @@ from server.operator._common import (
 
 router = APIRouter(prefix="/api/operator", tags=["operator-failure-patterns"])
 
+# The assist guardrail block-rate safety signal (TASK-10-02, D-060 layer 3).
+ASSIST_GUARDRAIL_BLOCK_RATE = "assist_guardrail_block_rate"
+
 
 def _is_failure_metric(metric: str) -> bool:
-    return metric.startswith("failure_mode:") or metric.startswith("branch:")
+    # Failure patterns (v0.4) + the assist guardrail block-rate safety signal (v0.5).
+    return (
+        metric.startswith("failure_mode:")
+        or metric.startswith("branch:")
+        or metric == ASSIST_GUARDRAIL_BLOCK_RATE
+    )
 
 
 @router.get("/failure-patterns", response_model=ViewResponse)