release: v0.13.1 — mic chart defaults to psi (matches PDF)

v0.13.0 shipped the mic_unit_pref default as "dBL", which made the
website chart's mic axis inconsistent with the PDF report (which
renders psi).  Original brief was always "psi on charts, dBL on
peaks" — I implemented the default backwards.  Operator caught it
within an hour of rollout.

Same-day patch:
- backend/models.py: default "dBL" → "psi"
- migrate_add_mic_unit_pref.py: idempotent across both fresh DB
  ("add column with psi default") and v0.13.0 upgrade ("flip dBL
  rows to psi").  One-row table, freshness assumed.
- backend/routers/settings.py: GET/PUT fallback "dBL" → "psi"
- templates/settings.html: dropdown's `selected` flag moves to psi
  + reorders options + relabels with "(matches PDF report)" hint
- backend/static/event-modal.js: module-level fallback + branch
  conditions flip to make psi the unset/error default

Includes the "Captured at" → "Time received" relabel from earlier
in the day (already-shipped commit 43c804d) rolled into the
release notes.

Migration is idempotent + safe to re-run; rolled out on the dev
container during this commit's smoke test.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/routers/settings.py

@@ -294,7 +294,7 @@ def get_preferences(db: Session = Depends(get_db)):
         "calibration_warning_days": prefs.calibration_warning_days,
         "status_ok_threshold_hours": prefs.status_ok_threshold_hours,
         "status_pending_threshold_hours": prefs.status_pending_threshold_hours,
-        "mic_unit_pref": prefs.mic_unit_pref or "dBL",
+        "mic_unit_pref": prefs.mic_unit_pref or "psi",
         "updated_at": prefs.updated_at.isoformat() if prefs.updated_at else None
     }
 
@@ -336,7 +336,7 @@ def update_preferences(
         "calibration_warning_days": prefs.calibration_warning_days,
         "status_ok_threshold_hours": prefs.status_ok_threshold_hours,
         "status_pending_threshold_hours": prefs.status_pending_threshold_hours,
-        "mic_unit_pref": prefs.mic_unit_pref or "dBL",
+        "mic_unit_pref": prefs.mic_unit_pref or "psi",
         "updated_at": prefs.updated_at.isoformat() if prefs.updated_at else None
     }