✨ feat: enhance soft-delete handling & boost pricing cache performance

Summary
-------
This commit unifies soft-delete behaviour across meta tables and
introduces an in-memory cache for model pricing look-ups to improve
throughput under high concurrency.

Details
-------
Soft-delete consistency
• PrefillGroup / Vendor / Model
  – Added `gorm.DeletedAt` field with `json:"-" gorm:"index"`.
  – Replaced plain `uniqueIndex` with partial unique indexes
    `uniqueIndex:<name>,where:deleted_at IS NULL`
    allowing duplicate keys after logical deletion while preserving
    uniqueness for active rows.
• Imports updated to include `gorm.io/gorm`.
• JSON output now hides `deleted_at`, matching existing tables.

High-throughput pricing cache
• model/pricing.go
  – Added thread-safe maps `modelEnableGroups` & `modelQuotaTypeMap`
    plus RW-mutex for O(1) access.
  – `updatePricing()` now refreshes these maps alongside `pricingMap`.
• model/model_extra.go
  – Rewrote `GetModelEnableGroups` & `GetModelQuotaType` to read from
    the new maps, falling back to automatic refresh via `GetPricing()`.

Misc
• Retained `RefreshPricing()` helper for immediate cache invalidation
  after admin actions.
• All modified files pass linter; no breaking DB migrations required
  (handled by AutoMigrate).

Result
------
– Soft-delete logic is transparent, safe, and allows record “revival”.
– Pricing-related queries are now constant-time, reducing CPU usage and
  latency under load.

model/prefill_group.go

@@ -4,6 +4,7 @@ import (
     "one-api/common"
 
     "gorm.io/datatypes"
+    "gorm.io/gorm"
 )
 
 // PrefillGroup 用于存储可复用的“组”信息，例如模型组、标签组、端点组等。
@@ -15,12 +16,13 @@ import (
 
 type PrefillGroup struct {
     Id          int            `json:"id"`
-    Name        string         `json:"name" gorm:"uniqueIndex;size:64;not null"`
+    Name        string         `json:"name" gorm:"size:64;not null;uniqueIndex:uk_prefill_name,where:deleted_at IS NULL"`
     Type        string         `json:"type" gorm:"size:32;index;not null"`
     Items       datatypes.JSON `json:"items" gorm:"type:json"`
     Description string         `json:"description,omitempty" gorm:"type:varchar(255)"`
     CreatedTime int64          `json:"created_time" gorm:"bigint"`
     UpdatedTime int64          `json:"updated_time" gorm:"bigint"`
+    DeletedAt   gorm.DeletedAt `json:"-" gorm:"index"`
 }
 
 // Insert 新建组