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/static/event-modal.js

@@ -46,7 +46,7 @@
     const MIC_DBL_FLOOR = 60;
 
     let _charts = {};                // ch → Chart instance
-    let _micUnitPref = 'dBL';        // refreshed via fetch on first chart render
+    let _micUnitPref = 'psi';        // refreshed via fetch on first chart render
     let _micUnitPrefLoaded = false;  // one-shot fetch guard
 
     function _esc(s) {
@@ -294,10 +294,10 @@
             const r = await fetch('/api/settings/preferences');
             if (r.ok) {
                 const prefs = await r.json();
-                _micUnitPref = prefs.mic_unit_pref === 'psi' ? 'psi' : 'dBL';
+                _micUnitPref = prefs.mic_unit_pref === 'dBL' ? 'dBL' : 'psi';
             }
         } catch (e) {
-            // Network error → silent fall back to default 'dBL'.
+            // Network error → silent fall back to default 'psi'.
         }
         _micUnitPrefLoaded = true;
         return _micUnitPref;