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/metrics.go

@@ -0,0 +1,94 @@
+package model
+
+import "time"
+
+// SummaryStats holds the top-level aggregate across all sessions.
+type SummaryStats struct {
+	TotalSessions   int
+	TotalPrompts    int
+	TotalAPICalls   int
+	TotalDurationSecs int64
+	ActiveDays      int
+
+	InputTokens           int64
+	OutputTokens          int64
+	CacheCreation5mTokens int64
+	CacheCreation1hTokens int64
+	CacheReadTokens       int64
+	TotalBilledTokens     int64
+
+	EstimatedCost float64
+	ActualCost    *float64
+	CacheSavings  float64
+	CacheHitRate  float64
+
+	CostPerDay     float64
+	TokensPerDay   int64
+	SessionsPerDay float64
+	PromptsPerDay  float64
+	MinutesPerDay  float64
+}
+
+// DailyStats holds metrics for a single calendar day.
+type DailyStats struct {
+	Date            time.Time
+	Sessions        int
+	Prompts         int
+	APICalls        int
+	DurationSecs    int64
+	InputTokens     int64
+	OutputTokens    int64
+	CacheCreation5m int64
+	CacheCreation1h int64
+	CacheReadTokens int64
+	EstimatedCost   float64
+	ActualCost      *float64
+}
+
+// ModelStats holds aggregated metrics for a single model.
+type ModelStats struct {
+	Model           string
+	APICalls        int
+	InputTokens     int64
+	OutputTokens    int64
+	CacheCreation5m int64
+	CacheCreation1h int64
+	CacheReadTokens int64
+	EstimatedCost   float64
+	SharePercent    float64
+	TrendDirection  int // -1, 0, +1 vs previous period
+}
+
+// ProjectStats holds aggregated metrics for a single project.
+type ProjectStats struct {
+	Project        string
+	Sessions       int
+	Prompts        int
+	TotalTokens    int64
+	EstimatedCost  float64
+	TrendDirection int
+}
+
+// HourlyStats holds prompt/session counts for one hour of the day.
+type HourlyStats struct {
+	Hour     int
+	Prompts  int
+	Sessions int
+	Tokens   int64
+}
+
+// WeeklyStats holds metrics for one calendar week.
+type WeeklyStats struct {
+	WeekStart    time.Time
+	Sessions     int
+	Prompts      int
+	TotalTokens  int64
+	DurationSecs int64
+	EstimatedCost float64
+}
+
+// PeriodComparison holds current and previous period data for delta computation.
+type PeriodComparison struct {
+	Current  SummaryStats
+	Previous SummaryStats
+}