feat(stats): add daily statistics aggregation job and model

- Create `DailyStat` model for immutable daily metrics including
  request counts, tokens, latency, and top models.
- Implement `DailyStatsJob` to aggregate `log_records` from the previous
  day, running daily at 00:05 UTC.
- Register database migrations and schedule the job in the server.
- Add `last7d` and `last30d` period support to stats handler.

internal/model/daily_stat.go

@@ -0,0 +1,42 @@
+package model
+
+import "time"
+
+// TopModelStat represents a single model's statistics in the top models list
+type TopModelStat struct {
+	Model    string `json:"model"`
+	Requests int64  `json:"requests"`
+	Tokens   int64  `json:"tokens"`
+}
+
+// DailyStat stores immutable daily aggregated statistics.
+// Once generated by the cron job, a row is never modified.
+// Design principles:
+// - Immutable: once generated, never modified
+// - Store raw aggregates, compute derived values (error_rate, avg_latency) at read time
+// - One row per day, ~365 rows/year
+type DailyStat struct {
+	Date string `gorm:"primaryKey;size:10" json:"date"` // "2006-01-02" (UTC)
+
+	// Request counts
+	Requests int64 `json:"requests"`
+	Success  int64 `json:"success"`
+	Failed   int64 `json:"failed"`
+
+	// Token counts
+	TokensIn  int64 `json:"tokens_in"`
+	TokensOut int64 `json:"tokens_out"`
+
+	// Latency (for avg calculation: avg = LatencySumMs / Requests)
+	LatencySumMs int64 `json:"latency_sum_ms"`
+
+	// Top models (JSON array of TopModelStat)
+	TopModels string `gorm:"type:text" json:"top_models"`
+
+	CreatedAt time.Time `json:"created_at"`
+}
+
+// TableName specifies the table name for DailyStat
+func (DailyStat) TableName() string {
+	return "daily_stats"
+}