settings: add mic_unit_pref for event-report chart

New UserPreferences field controls the mic channel's unit on the
SFM event-detail modal's waveform chart only.  "dBL" default,
"psi" alternate.  Peaks everywhere else (tables, KPI tiles, modal
summary) stay in dBL regardless — this is strictly a chart-axis
preference.

Surfaced as a single dropdown on Settings → General, below the
auto-refresh interval.

Setting up the storage half ahead of the chart port in the next
commit, so the chart can read the value from /api/settings/preferences
on first render instead of needing a follow-up wiring pass.

Includes idempotent backend/migrate_add_mic_unit_pref.py for fleets
already on an older schema.

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

backend/migrate_add_mic_unit_pref.py

@@ -0,0 +1,56 @@
+#!/usr/bin/env python3
+"""
+Database migration: Add mic_unit_pref column to user_preferences.
+
+Adds a single field controlling the mic channel's unit on the event-
+report waveform chart in the SFM event detail modal.  "dBL" (default)
+or "psi".  Peaks and KPI tiles elsewhere are always dBL regardless.
+
+Idempotent — safe to re-run.
+"""
+
+import sqlite3
+from pathlib import Path
+
+
+def migrate():
+    possible_paths = [
+        Path("data/seismo_fleet.db"),
+        Path("data/sfm.db"),
+        Path("data/seismo.db"),
+    ]
+    db_path = next((p for p in possible_paths if p.exists()), None)
+    if db_path is None:
+        print(f"Database not found in any of: {[str(p) for p in possible_paths]}")
+        print("Will be created with the new column when models.py initialises.")
+        return
+
+    print(f"Using database: {db_path}")
+    conn = sqlite3.connect(db_path)
+    cur = conn.cursor()
+
+    cur.execute("PRAGMA table_info(user_preferences)")
+    existing = {row[1] for row in cur.fetchall()}
+
+    if "mic_unit_pref" in existing:
+        print("mic_unit_pref already exists — nothing to do.")
+        conn.close()
+        return
+
+    cur.execute(
+        "ALTER TABLE user_preferences "
+        "ADD COLUMN mic_unit_pref TEXT DEFAULT 'dBL'"
+    )
+    # Backfill the single row that should exist (id=1) to the default,
+    # in case the column ends up NULL on existing rows.
+    cur.execute(
+        "UPDATE user_preferences SET mic_unit_pref = 'dBL' "
+        "WHERE mic_unit_pref IS NULL"
+    )
+    conn.commit()
+    conn.close()
+    print("Added mic_unit_pref to user_preferences (default 'dBL').")
+
+
+if __name__ == "__main__":
+    migrate()