feat(events): add ending soon badges

[keeltrace]

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Fixes #184.

What changed

• Calculate an endingSoon state alongside the existing happening state for event occurrences.
• Treat an event as ending soon only while it is running and less than 25% of its original duration remains.
• Surface that state on the parent event when any current occurrence is ending soon.
• Replace the NOW badge with ENDING SOON during that final quarter in event cards, the favorites-events card, and occurrence detail.
• Keep the calculated fields optional so existing event fixtures / serialized data do not need a schema migration.

Boundary behavior

The comparison is deliberately strict to match the issue wording: exactly 25% remaining still shows NOW; the ENDING SOON state begins immediately after that threshold. Before start, at/after end, invalid times, and zero/negative durations do not qualify.

Tests

Added focused unit coverage for:

• 1–5 PM event at 4:30 PM → ending soon
• exactly 25% remaining → not ending soon
• immediately below 25% → ending soon
• before start / at end / after end → not ending soon
• invalid and non-positive durations → not ending soon

The repository's pull-request workflow will run its normal Bun lint, unit-test, and production-build gates on this branch.