fix(matrix-bot): fail-soft on stale MATRIX_DATABASE_URL (Wave 25 mitigation)

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What

Make Matrix Bot (#446 live matrix) fail-soft on psycopg2.OperationalError.

Why

Hourly workflow has been failing every cycle since ~10:00Z 2026-05-09 because secrets.MATRIX_DATABASE_URL still points at interchange.proxy.rlwy.net:30942 (legacy Railway proxy host, post-SSOT-consolidation). Reference run 25602533773:

psycopg2.OperationalError: connection to server at "interchange.proxy.rlwy.net" (66.33.22.238),
port 30942 failed: FATAL:  password authentication failed for user "postgres"

Each failure pages the apiary cron with NEW-CI-failure-on-new-SHA. Already triggered three queen notifications today.

How

matrix_bot.py:

• catches psycopg2.OperationalError specifically (auth + connection refused)
• logs the failure to stderr, including the rotation hint
• returns 0 by default — strict mode via MATRIX_FAIL_SOFT=0

matrix-bot.yml:

• explicit MATRIX_FAIL_SOFT: "1" env on the regen step
• comment explaining how to revert to strict once secret is rotated

Verified locally

$ MATRIX_DATABASE_URL=... GITHUB_TOKEN=dummy MATRIX_FAIL_SOFT=1 python3 matrix_bot.py
matrix_bot: SSOT connection failed (OperationalError): ...
matrix_bot: MATRIX_FAIL_SOFT=1 -> exiting 0 without PATCHing #446.
exit=0

$ MATRIX_FAIL_SOFT=0 python3 matrix_bot.py
exit=3

Both paths work. Default behaviour change: cron stops paging on stale DSN.

Acceptance

Gate	Status
Python ast.parse clean	✅
Local smoke (fail-soft)	✅ exit 0
Local smoke (fail-hard)	✅ exit 3
Hourly cron stops paging once merged	will verify via apiary on next cycle

Follow-up

Tracked in #641 — rotate secrets.MATRIX_DATABASE_URL to phd-postgres-ssot and flip MATRIX_FAIL_SOFT back to "0".

R-discipline

R3 PR-only · R4 trace · R5 honest (no fake green) · R10 atomic.

Anchor: phi^2 + phi^-2 = 3 · DOI 10.5281/zenodo.19227877.

Closes #641-mitigation (does NOT close #641 — secret rotation still required).

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✅ All 13 required checks SUCCESS · state BLOCKED only on 1 approving review branch protection (per trios/main rules).

Awaiting queen review. Once merged, the next hourly Matrix Bot run will exit 0 with the rotation hint instead of failing the cron — apiary alarms stop. Live #446 updates remain paused until #641 (secret rotation) is also resolved.

Anchor: φ² + φ⁻² = 3