feat(license): add BSL license enforcement with device limit indicator

- Add LICENSE_DEVICES env var (default 250, matches BSL 1.1 free tier)
- Add /api/settings/license endpoint returning device count vs limit
- Header shows flashing red "502/500 licensed" badge when over limit
- About page shows license tier, device count, and over-limit warning
- Nothing is crippled — all features work regardless of device count
- Bump version to 9.7.1

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/lib/settingsApi.ts

@@ -45,3 +45,15 @@ export async function testSMTPSettings(data: {
   const res = await api.post('/api/settings/smtp/test', data)
   return res.data
 }
+
+export interface LicenseStatus {
+  licensed_devices: number
+  actual_devices: number
+  over_limit: boolean
+  tier: 'free' | 'commercial'
+}
+
+export async function getLicenseStatus(): Promise<LicenseStatus> {
+  const res = await api.get('/api/settings/license')
+  return res.data
+}