fix: coerce numeric/decimal columns in repository mappers (refs #241)

node-postgres returns numeric/decimal columns as JavaScript strings,
but the TypeScript interfaces for MaintenanceRecord and OwnershipCost
declare numeric fields as number. The mappers were passing values
through raw, breaking type-safe arithmetic and display (e.g. the
amount column on the vehicle summary screen was empty until the
recent frontend workaround in PR #240, and OwnershipCostsList silently
no-ops toLocaleString on the string).

Backend
- mapMaintenanceRecord: coerce cost via Number() when non-null.
- ownership-costs mapRow: coerce amount via Number().

Frontend (remove now-redundant workarounds)
- MaintenanceRecordsList: drop Number() coercion on cost and
  odometerReading; use the number values directly.
- VehicleDetailPage / VehicleDetailMobile: revert the PR #240 cost
  coercion to the simple typeof number guard now that the backend
  honors the type.

Scope notes
- Other repositories with the same pattern (stations, community-stations,
  fuel-logs enhanced methods) are tracked separately because they have
  unclear downstream consumers and warrant their own investigation.

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

frontend/src/features/maintenance/components/MaintenanceRecordsList.tsx

@@ -115,16 +115,16 @@ export const MaintenanceRecordsList: React.FC<MaintenanceRecordsListProps> = ({
                       {categoryDisplay} ({subtypeCount})
                     </Typography>
                     <Stack direction="row" spacing={1} flexWrap="wrap" sx={{ mt: 1 }}>
-                      {record.odometerReading && (
+                      {record.odometerReading != null && (
                         <Chip
-                          label={`${Number(record.odometerReading).toLocaleString()} miles`}
+                          label={`${record.odometerReading.toLocaleString()} miles`}
                           size="small"
                           variant="outlined"
                         />
                       )}
-                      {record.cost && (
+                      {record.cost != null && (
                         <Chip
-                          label={`$${Number(record.cost).toFixed(2)}`}
+                          label={`$${record.cost.toFixed(2)}`}
                           size="small"
                           color="primary"
                           variant="outlined"

frontend/src/features/vehicles/mobile/VehicleDetailMobile.tsx

@@ -142,9 +142,7 @@ export const VehicleDetailMobile: React.FC<VehicleDetailMobileProps> = ({
         secondaryParts.push(new Date(rec.date).toLocaleDateString());
         secondaryParts.push('Maintenance');
         const secondary = secondaryParts.join(' • ');
-        // Backend returns numeric/decimal columns as strings via node-postgres; coerce.
-        const costNum = rec.cost != null ? Number(rec.cost) : NaN;
-        const amount = Number.isFinite(costNum) ? `$${costNum.toFixed(2)}` : undefined;
+        const amount = typeof rec.cost === 'number' ? `$${rec.cost.toFixed(2)}` : undefined;
         list.push({
           id: rec.id,
           type: 'Maintenance',

frontend/src/features/vehicles/pages/VehicleDetailPage.tsx

@@ -148,9 +148,7 @@ export const VehicleDetailPage: React.FC = () => {
         if (subtypeText) parts.push(subtypeText);
         if (rec.shopName) parts.push(rec.shopName);
         const summary = parts.join(' • ');
-        // Backend returns numeric/decimal columns as strings via node-postgres; coerce.
-        const costNum = rec.cost != null ? Number(rec.cost) : NaN;
-        const amount = Number.isFinite(costNum) ? `$${costNum.toFixed(2)}` : undefined;
+        const amount = typeof rec.cost === 'number' ? `$${rec.cost.toFixed(2)}` : undefined;
         list.push({ id: rec.id, type: 'Maintenance', date: rec.date, summary, amount });
       }
     }