Fix refresh scheduling

[Finesssee]

Summary

• anchor fixed refresh cadence to scheduled ticks and skip missed ticks without overlap
• return quota refresh completion before a single-flight local-cost enrichment tail
• refresh provider data when provider enablement changes while visual settings and reorder remain cache-only

Thermo-nuclear review

Approved after replacing queued enrichment tasks with one single-flight tail and deleting the obsolete cache-age scheduler model/tests. No new scheduler or frontend state layer was added.

Validation

• shared Rust tests and Clippy -D warnings
• Tauri tests and Clippy -D warnings
• full Vitest suite and frontend build
• git diff --check