feat: add domain model types for session metrics and statistics

Define the core data structures that flow through the entire pipeline:

- model/session.go: SessionStats (per-session aggregates including
  token counts across 5 categories — input, output, cache_write_5m,
  cache_write_1h, cache_read), APICall (deduplicated by message.id,
  keyed to the final billed usage), and ModelUsage (per-model
  breakdown within a session). Tracks subagent relationships via
  IsSubagent/ParentSession fields.

- model/metrics.go: Higher-order aggregate types — SummaryStats
  (top-level totals with per-active-day rates for cost, tokens,
  sessions, and minutes), DailyStats/HourlyStats/WeeklyStats
  (time-bucketed views), ModelStats (cross-session model comparison
  with share percentages), ProjectStats (per-project ranking), and
  PeriodComparison (current vs previous period for delta display).

- model/budget.go: BudgetStats with plan ceiling, custom budget,
  burn rate, and projected monthly spend for the budget tab.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/model/budget.go

@@ -0,0 +1,12 @@
+package model
+
+// BudgetStats holds budget tracking and forecast data.
+type BudgetStats struct {
+	PlanCeiling       float64
+	CustomBudget      *float64
+	CurrentSpend      float64
+	DailyBurnRate     float64
+	ProjectedMonthly  float64
+	DaysRemaining     int
+	BudgetUsedPercent float64
+}