Make verification execution mode requirement-driven instead of enum hardcoding

[madebygps]

Problem

Sync vs async execution mode is currently hardcoded by submission type. This makes behavior changes require code edits and increases coupling between content and runtime.

Scope

• Define execution mode as requirement-level config
• Update routing to use requirement config instead of type-only switch
• Preserve existing behavior by default during migration

Acceptance criteria

• Execution mode is configurable per requirement
• Current requirements keep existing runtime behavior unless explicitly changed
• Tests cover config-driven dispatch