fix: usage buckets should count actual last-30-day usage

[forestileao]

Problem

The event bucket and agent token bucket currently use proportional leak logic. Each day, the stored bucket level decreases by limit / 30.

That makes both the budget calculation and UI wrong. If an organization uses 500,000 events in one day, the next day it incorrectly appears to recover 500000 / 30 capacity. Instead, the buckets should reflect actual usage in the last 30 days.

Expected behavior

For both buckets:

bucket level = sum of actual usage from the last 30 days

Example:

• Day 1: org uses 500,000 events.
• Days 2-30: event bucket remains 500,000 if no additional usage happens.
• Day 31: day 1 leaves the 30-day window, so the event bucket decreases by whatever was used on day 1.
• If no usage happened on days 2-31, the event bucket becomes 0.

The same logic applies to the agent token bucket.

Unlimited plans must still track bucket usage and show used / ∞.

The usage UI should display these actual last-30-day bucket values.

Acceptance criteria

• Event bucket does not decrease on day 2 after a day-1 spike.
• Event bucket decreases on day 31 by the actual day-1 event count.
• Agent token bucket follows the same last-30-day bucket logic.
• Neither bucket decreases by limit / 30.
• Multiple daily buckets with different values sum correctly.
• Unlimited plans still track and display bucket usage.
• Duplicate event IDs and duplicate agent-token usage IDs do not double count.
• SuperPlane usage UI displays corrected bucket values as used / limit or used / ∞.

Notes

Daily buckets should use UTC day boundaries. This issue is scoped to bucket accounting and UI correctness, not adding new runtime blocking behavior.