feat: add Services and Logs modules (systemctl + journalctl via SSH)

Backend:
- modules/services: list, status, start/stop/restart systemctl services
  with pct exec support for LXC targets
- modules/logs: journalctl unit listing + WebSocket live streaming
  (direct SSH connection, journalctl -f, graceful teardown on WS close)
- migrations/003: seed services and logs modules in DB
- main.go: register services.New() and logs.New() in module loader

Frontend:
- services.html: target selector, search/filter, services table with
  active state indicators and start/stop/restart buttons
- logs.html: target + unit selectors, live follow toggle, scrollable
  terminal output with 3000-line cap
- app.js: servicePage() and logsPage() Alpine components + navItems
- locales: services and logs i18n keys (fr + en)
- pages.css: services table, state dots, logs output styles

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/db/migrations/003_modules_extra.sql

@@ -0,0 +1,4 @@
+-- Migration 003 : Ajout des modules logs et services
+INSERT OR IGNORE INTO modules (id, name, description, version, is_core, is_enabled) VALUES
+    ('logs',     'Journaux',  'Consultation des journaux système via journalctl',    '1.0.0', 0, 1),
+    ('services', 'Services',  'Gestion des services systemd (start/stop/restart)',  '1.0.0', 0, 1);