feat: add beacon monitors with user and admin management
Backend:
- Migration: beacon_monitors table (binary_id PK, org/user FK, check_type, target_url, config, monitoring fields)
- Schema: BeaconMonitor with changeset (pattern-matched port validation, check_type inclusion)
- Query: BeaconMonitorQuery — composable scopes (by org, by user, by type, enabled, ordered, preloaded)
- Context: Towerops.Beacons — CRUD, toggle, record_check_result (36/36 tests pass)
Frontend:
- User LiveView: /beacons — stream-based list, create/edit modals, toggle/delete actions
- Admin LiveView: /admin/beacons — cross-org view with preloads (20/20 tests pass)
- Form component: LiveComponent modal with validation
- Helpers: check_type badges, status indicators, response time formatting
- Router: user routes + admin route configured
- Nav: sidebar + mobile links added
- 30+ gettext translations
Verification: compile ✓, format ✓, credo ✓, 56/56 new tests pass
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