feat(core): implement group-based routing and optimize sync

Replace direct provider linkage with group-based routing for Keys and
Providers. This allows for more flexible load balancing and tiering
strategies.

Changes include:
- Remove `ProviderID` from Key model and DTO
- Add `Group` field to Key and Provider models
- Refactor Redis sync to use Hashes for O(1) partial updates
- Update API handlers to perform incremental syncs

BREAKING CHANGE: Key API payload no longer accepts `provider_id`. Redis
configuration storage format has changed from JSON strings to Hashes.

internal/model/models.go

@@ -17,16 +17,16 @@ type Provider struct {
 	Type    string `gorm:"not null" json:"type"` // openai, anthropic, etc.
 	BaseURL string `json:"base_url"`
 	APIKey  string `json:"api_key"`
+	Group   string `gorm:"default:'default'" json:"group"` // routing group/tier
 }
 
 type Key struct {
 	gorm.Model
-	ProviderID *uint     `json:"provider_id"`
-	Provider   *Provider `json:"-"`
-	KeySecret  string    `gorm:"not null" json:"key_secret"`
-	Balance    float64   `json:"balance"`
-	Status     string    `gorm:"default:'active'" json:"status"` // active, suspended
-	Weight     int       `gorm:"default:10" json:"weight"`
+	KeySecret string  `gorm:"not null" json:"key_secret"`
+	Group     string  `gorm:"default:'default'" json:"group"` // routing group/tier
+	Balance   float64 `json:"balance"`
+	Status    string  `gorm:"default:'active'" json:"status"` // active, suspended
+	Weight    int     `gorm:"default:10" json:"weight"`
 }
 
 type Model struct {