Add capacity forecast client method

[cniackz]

Summary

• Add CapacityForecast struct with storage capacity predictions
• Add CapacityForecast() client method calling GET /admin/v4/capacity-forecast
• Follows existing ServerInfo pattern

Struct fields (15)

Field	Type	Description
CurrentUsedBytes	uint64	Current used storage
CurrentTotalBytes	uint64	Total usable storage
CurrentUsedPercent	float64	Used percentage (raw, client formats)
DaysUntil80Pct	*float64	Days until 80% — nil if no prediction; may be negative if already past
DaysUntil90Pct	*float64	Days until 90% — same semantics
DaysUntil100Pct	*float64	Days until full — same semantics
GrowthRatePerDay	int64	Average bytes/day
DataPointCount	int	Daily snapshots collected
MinDaysUntilFull	*float64	Worst case based on largest single-day delta — nil if no prediction
RSquared	float64	Confidence: goodness of fit (0–1)
Variance	float64	Variance of daily deltas
DayMinDelta	float64	Smallest daily change in usedFraction (can be negative)
DayMaxDelta	float64	Largest daily change in usedFraction
RecentGrowthRatePerDay	float64	14-day window growth rate
RecentDaysUntilFull	*float64	14-day window days-until-full (nil if no prediction)

Design notes

• Days-until fields are *float64 (not float64 with sentinel) so nil cleanly represents "no prediction possible". A concrete value may be negative when the threshold was already crossed in the past — magnitude is meaningful.
• All numeric fields return raw precision; the client decides formatting.

Related PRs

• eos: https://github.com/miniohq/eos/pull/4082
• ec: https://github.com/miniohq/ec/pull/484