Context
Prepare nightly tally for Phase 2 HBI integration by refactoring host update logic into compartmentalized sections. This creates clean separation between legacy HBI database queries and future normalized event processing while maintaining zero operational risk.

The refactoring introduces an Unleash feature flag (swatch.swatch-tally.legacy-host-updates) that controls which host update mechanism is used, with the default being a no-op to ensure toggling the flag away from legacy produces no immediate changes.

Acceptance Criteria

• Zero Risk Toggle: When Unleash flag is toggled away from legacy mode, nothing happens (no-op behavior)
• Compartmentalized Logic: Nightly tally host update logic separated into distinct doLegacyStuff() and doNewStuff() sections
• Unleash Flag Integration: Feature flag swatch.swatch-tally.legacy-host-updates controls which host updating mechanism is active
• No-Op Default: New mechanism starts as no-op to prevent accidental activation
• Parallel Work Enablement: Teams can work on new logic without merge conflicts with existing code
• Existing Functionality Preserved: All current nightly tally behavior remains unchanged

Unleash Flag Setup
The Unleash flag setup process is minimal since infrastructure already exists from SWATCH-3018. Setup instructions documented in PR #3825 - uses existing swatch-unleash-import service for automatic flag creation across environments.