Add per-meter MQTT value scale + expose device topic/scale in Meters UI

The water-softener totaliser publishes litres but the water category is
m3 — without a conversion factor its readings would land in the DB
1000x too high. Also, mqtt_topic was only settable via direct SQL since
the Meters form never exposed it.

frontend/src/pages/Meters.tsx

@@ -10,6 +10,7 @@ const emptyForm = {
   id: 0, category_id: 0, parent_meter_id: '' as number | '', name: '', location: '',
   serial_number: '', install_date: '', notes: '', active: true,
   gas_volume_unit: '' as 'm3' | 'ft3' | '', gas_calorific_value: '' as number | '',
+  mqtt_topic: '', mqtt_value_scale: 1 as number | '',
 }
 
 export default function Meters() {
@@ -54,6 +55,7 @@ export default function Meters() {
       install_date: m.install_date ? m.install_date.slice(0, 10) : '', notes: m.notes || '',
       active: m.active,
       gas_volume_unit: m.gas_volume_unit || '', gas_calorific_value: m.gas_calorific_value != null ? Number(m.gas_calorific_value) : '',
+      mqtt_topic: m.mqtt_topic || '', mqtt_value_scale: m.mqtt_value_scale != null ? Number(m.mqtt_value_scale) : 1,
     })
     setImageFile(null)
   }
@@ -76,6 +78,8 @@ export default function Meters() {
         active: form.active,
         gas_volume_unit: isGas && form.gas_volume_unit ? form.gas_volume_unit : null,
         gas_calorific_value: isGas && form.gas_calorific_value !== '' ? form.gas_calorific_value : null,
+        mqtt_topic: form.mqtt_topic.trim() || null,
+        mqtt_value_scale: form.mqtt_value_scale === '' ? 1 : form.mqtt_value_scale,
       }
       const saved = form.id ? await api.updateMeter(form.id, body) : await api.createMeter(body)
       if (imageFile) await api.uploadMeterImage(saved.id, imageFile)
@@ -220,6 +224,21 @@ export default function Meters() {
               </div>
             )}
 
+            <div className="field-row">
+              <div className="field">
+                <label>MQTT topic (device feed)</label>
+                <input type="text" value={form.mqtt_topic} onChange={e => setForm({ ...form, mqtt_topic: e.target.value })}
+                  placeholder="e.g. utilities/water-softener/total_usage_l" />
+                <div className="field-hint">Leave blank for a manually-read meter. If set, readings are ingested automatically from the shared MQTT broker.</div>
+              </div>
+              <div className="field">
+                <label>Device value scale</label>
+                <input type="number" step="0.000001" value={form.mqtt_value_scale}
+                  onChange={e => setForm({ ...form, mqtt_value_scale: e.target.value === '' ? '' : parseFloat(e.target.value) })} />
+                <div className="field-hint">Multiplied onto the raw published value, e.g. 0.001 if the device publishes litres but this meter's unit is m³.</div>
+              </div>
+            </div>
+
             <div className="field-row">
               <div className="field">
                 <label>Parent meter (sub-metering)</label>

frontend/src/types.ts

@@ -40,6 +40,8 @@ export interface Meter {
   reading_unit_label: string
   gas_volume_unit: 'm3' | 'ft3' | null
   gas_calorific_value: number | null
+  mqtt_topic: string | null
+  mqtt_value_scale: number
   parent_meter_id: number | null
   parent_name: string | null
   name: string