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/session.go

@@ -0,0 +1,55 @@
+package model
+
+import "time"
+
+// APICall represents one deduplicated API request (final state of a message.id).
+type APICall struct {
+	MessageID             string
+	Model                 string
+	Timestamp             time.Time
+	InputTokens           int64
+	OutputTokens          int64
+	CacheCreation5mTokens int64
+	CacheCreation1hTokens int64
+	CacheReadTokens       int64
+	ServiceTier           string
+	EstimatedCost         float64
+}
+
+// ModelUsage tracks per-model token usage within a session.
+type ModelUsage struct {
+	APICalls              int
+	InputTokens           int64
+	OutputTokens          int64
+	CacheCreation5mTokens int64
+	CacheCreation1hTokens int64
+	CacheReadTokens       int64
+	EstimatedCost         float64
+}
+
+// SessionStats holds aggregated metrics for a single session file.
+type SessionStats struct {
+	SessionID     string
+	Project       string
+	ProjectPath   string
+	FilePath      string
+	IsSubagent    bool
+	ParentSession string
+	StartTime     time.Time
+	EndTime       time.Time
+	DurationSecs  int64
+
+	UserMessages int
+	APICalls     int
+
+	InputTokens           int64
+	OutputTokens          int64
+	CacheCreation5mTokens int64
+	CacheCreation1hTokens int64
+	CacheReadTokens       int64
+
+	Models map[string]*ModelUsage
+
+	EstimatedCost float64
+	CacheHitRate  float64
+}