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/dto/key.go

@@ -2,9 +2,9 @@ package dto
 
 // KeyDTO defines payload for key creation/update.
 type KeyDTO struct {
-    ProviderID uint    `json:"provider_id"`
-    KeySecret  string  `json:"key_secret"`
-    Balance    float64 `json:"balance"`
-    Status     string  `json:"status"`
-    Weight     int     `json:"weight"`
+	Group     string  `json:"group"`
+	KeySecret string  `json:"key_secret"`
+	Balance   float64 `json:"balance"`
+	Status    string  `json:"status"`
+	Weight    int     `json:"weight"`
 }

internal/dto/provider.go

@@ -2,8 +2,9 @@ package dto
 
 // ProviderDTO defines inbound payload for provider creation/update.
 type ProviderDTO struct {
-    Name    string `json:"name"`
-    Type    string `json:"type"`
-    BaseURL string `json:"base_url"`
-    APIKey  string `json:"api_key"`
+	Name    string `json:"name"`
+	Type    string `json:"type"`
+	BaseURL string `json:"base_url"`
+	APIKey  string `json:"api_key"`
+	Group   string `json:"group"`
 }