SR-187: Ban datetime.utcnow() — enforce UTC-aware datetimes

[Delqhi]

SR-187: Ban datetime.utcnow() — Enforce UTC-aware datetimes

Why this is critical

Python 3.12 deprecated datetime.utcnow() (DeprecationWarning), will remove in 3.14. More critically for our domain: naive datetimes are ambiguous when compared against UTC-aware datetimes. We found 6 production instances:

survey/captcha/fallback_chain.py:189,197    # Survey captcha timestamps
survey/daemon/answer_engine.py:1005         # Answer engine audit log
survey/daemon/survey_agent_graph.py:173,235 # LangGraph state: started_at, completed_at

These timestamps are compared against UTC-aware DB timestamps — silent off-by-tz is possible.

Solution

Add datetime.utcnow to scripts/check_banned_patterns.py banlist. One PR refactors the 6 sites:

# OLD
datetime.utcnow().isoformat()

# NEW
from datetime import datetime, timezone
datetime.now(timezone.utc).isoformat()

Acceptance Criteria

• check_banned_patterns.py extended with regex for datetime.utcnow()
• All 6 instances refactored
• CI enforces: any new datetime.utcnow() in production code fails path-guard
• Tests confirm: all timestamps are datetime(..., tzinfo=timezone.utc)
• Docs: add to AGENTS.md → "datetime hygiene"

Priority

critical — blocks Python 3.14 upgrade, creates silent timezone bugs today.

[Delqhi]

Doku-Follow-up: PR #192 — AGENTS.md 'datetime hygiene' section

Letzter offener Punkt aus der Acceptance-Liste:

• Docs: AGENTS.md 'datetime hygiene' section

Inhalt (2 Edits in survey-cli/AGENTS.md):

1. ARCHÄOLOGIE-TSUNAMI Schritt 3 BANNED-Patterns-Liste erwähnt jetzt datetime.utcnow() mit Forward-Ref auf die neue Sektion.
2. Neue Sub-Sektion ### Python / Datetime Hygiene (SR-187, Issue #186) unter EXPLICITE VERBOTE mit: Warum-banned, Pflicht-Pattern (datetime.now(timezone.utc)), Wire-Format-Entscheidung (Z vs +00:00), Enforcement-Path, Test-Location.

Mit Merge von #191 + #192 sind alle Acceptance-Items aus #186 erfüllt.