refactor(stats): remove daily stats aggregation

Remove the DailyStatsJob, DailyStat model, and associated database
migrations. This eliminates the pre-aggregation layer and updates the
dashboard handler to remove dependencies on the daily_stats table.

internal/model/daily_stat.go

@@ -1,42 +0,0 @@
-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"
-}