feat(settings): Settings page with email, webhooks, notifications, profile [FASE-8]

- SystemConfig model + migration b033 for runtime key-value config
- GET/PATCH /system/email-config + POST /system/email-test (admin only)
- email_service reads SMTP config from DB (overrides .env)
- Webhooks now accessible to red_lead/blue_lead + admin
- GET /users/me already existed; /users/me/preferences already working
- SettingsPage with 4 role-aware tabs:
  * Profile & Jira: jira_account_id, user info
  * Notifications: role-specific email/in-app toggles (12 prefs)
  * Webhooks: full CRUD + test ping (leads + admin)
  * Email/SMTP: enable toggle, server config, test email (admin only)
- Added /settings route (all authenticated users)
- Settings link added to Sidebar

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

frontend/src/components/Sidebar.tsx

@@ -50,6 +50,7 @@ const mainLinks: NavItem[] = [
   { to: "/compliance", label: "Compliance", icon: ShieldCheck },
   { to: "/comparison", label: "Comparison", icon: GitCompareArrows, roles: ["admin", "red_lead", "blue_lead", "viewer"] },
   { to: "/reports", label: "Reports", icon: BarChart3 },
+  { to: "/settings", label: "Settings", icon: Settings },
 ];
 
 const systemLinks: NavItem[] = [