fix(activity-feed): paint cached history before waiting on network call

[wrjones104]

Summary

• Navigating to the Activity feed blocked on getUserTrackedSlots() before reading already-synced local history, adding a 1-2s stall to every transition even though the data was on-device.
• HistoryViewModel.refreshAllHistory() now paints from the local DB immediately when tracked-slot metadata from a prior load is already in memory, then reconciles silently once the network call returns. First-ever load in a session is unaffected since there's no warm metadata yet to paint from.
• Folded this fix into the still-unpublished v1.6.23 changelog entry (backend/app/data/changelog.json) alongside the item-group filtering/ignore-menu changes, and regenerated android/CHANGELOG.md / backend/CHANGELOG.md.

Test plan

• Manually verify: open Activity feed from Rooms, SlotDetail, and the bottom nav tab after a warm session — history should paint instantly with no spinner stall.
• Manually verify: first load in a fresh session (no warm metadata) still works and shows a brief loading state as before.
• python scripts/generate_changelog.py --check passes.

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