refactor(bayn): use Effect platform and date boundaries

[gregkonush]

Summary

• Uses Effect platform services and DateTime boundaries in HTTP, schema, OBSERVE, and database tests.
• Keeps this native stack layer independently reviewable at 719 changed lines.
• Bases directly on codex/bayn-effect-tsgo-test-idioms-final so GitHub shows only this layer.

Related Issues

None

Testing

• bunx tsc --noEmit --project services/bayn/tsconfig.json
• bun run lint
• bun run lint:oxlint
• bun run lint:oxlint:type
• bun run build
• Focused tests for the touched subsystem were run while constructing the layer.

Breaking Changes

None

Checklist

• Testing section documents the exact validation performed.
• Screenshots are not applicable and Breaking Changes is filled in.
• Documentation and follow-ups are unchanged or represented by later stack layers.

[gregkonush]