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/models.py

@@ -136,8 +136,10 @@ class UserPreferences(Base):
     status_ok_threshold_hours = Column(Integer, default=12)
     status_pending_threshold_hours = Column(Integer, default=24)
     # Mic display units on the event-report waveform chart only — peaks
-    # and KPI tiles elsewhere are always dBL.  "dBL" (default) or "psi".
-    mic_unit_pref = Column(String, default="dBL")
+    # and KPI tiles elsewhere are always dBL.  "psi" (default — matches
+    # the PDF report) or "dBL".  Default flipped in v0.13.1 after
+    # operator feedback that the chart should mirror the PDF.
+    mic_unit_pref = Column(String, default="psi")
     updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)