security(secrets): any logged-in user can list and delete infrastructure hosts through /api/infrastructure/hosts #16426

Part of #16425 (Secrets page templates + admin-only infrastructure hosts umbrella).

Problem

Every route in autobot-backend/api/secrets.py requires admin (Depends(check_admin_permission)). But the infrastructure-host routes in autobot-backend/api/infrastructure.py do not: get_infrastructure_hosts (GET /api/infrastructure/hosts) and delete_infrastructure_host (DELETE /api/infrastructure/hosts/{id}) depend only on Depends(get_current_user). Any authenticated user can list every host's connection metadata (host, ports, username, os, capabilities, description) and delete any host — no admin check, no ownership check.

autobot-frontend/src/components/security/SecretsManager.vue merges these legacy hosts (via composables/security/useSecretsInfraApi.ts) into its list and renders the "Infrastructure Hosts" category with no role gate, so a non-admin user sees and can act on them through the Secrets page too.

The owner's decision: infrastructure hosts are admin-only.

Evidence

• autobot-backend/api/infrastructure.py: get_infrastructure_hosts and delete_infrastructure_host, both _user: Any = Depends(get_current_user)
• autobot-backend/api/secrets.py: every route uses Depends(check_admin_permission) for comparison
• autobot-frontend/src/composables/security/useSecretsInfraApi.ts
• autobot-frontend/src/components/security/SecretsManager.vue: credentialCategories includes infrastructure_host with no admin check found

Fix

• Add an admin dependency (matching api/secrets.py's pattern) to both get_infrastructure_hosts and delete_infrastructure_host.
• Frontend: hide the "Infrastructure Hosts" category and any hosts fetched via useSecretsInfraApi from non-admin users.

Acceptance criteria

• GET /api/infrastructure/hosts requires admin permission
• DELETE /api/infrastructure/hosts/{id} requires admin permission
• Non-admin frontend users never see the Infrastructure Hosts category or any legacy host entries
• A test confirms a non-admin gets 403 on both GET and DELETE, and an admin gets through on both

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