feat: Add custom MFA authentication flow with configurable enforcement

- Add authentication-flow.tf with complete MFA auth flow:
  - Identification -> Password -> MFA validation -> Session stages
  - Brute-force reputation policy binding
  - Evaluates policies on plan for user context

- Add configuration variables:
  - enable_mfa_flow: Toggle custom MFA flow (default: false)
  - mfa_enforcement: skip/configure/deny (default: configure)

- Fix existing issues:
  - rbac-groups.tf: parent -> parents (list)
  - source-google.tf: Use variables instead of deprecated sops
  - Google source now conditional (created only if credentials provided)

- Update README:
  - Document MFA enforcement levels
  - Add authentication-flow.tf to file structure
  - Explain Option 1 (Terraform) vs Option 2 (manual UI) for MFA setup

Security: Custom flow includes brute-force protection policy bound
at flow level, not just stage level.

security-policies.tf

@@ -99,7 +99,7 @@ resource "authentik_stage_authenticator_validate" "mfa_validation" {
   # - skip: Don't require MFA if not configured
   # - deny: Block login if MFA not configured (after enabling, users need MFA)
   # - configure: Force users to set up MFA if not configured
-  not_configured_action = "configure"
+  not_configured_action = var.mfa_enforcement
   
   # Supported authenticator types
   device_classes = [