✨ 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/model_meta.go

@@ -36,7 +36,7 @@ type BoundChannel struct {
 
 type Model struct {
     Id          int            `json:"id"`
-    ModelName   string         `json:"model_name" gorm:"uniqueIndex;size:128;not null"`
+    ModelName   string         `json:"model_name" gorm:"size:128;not null;uniqueIndex:uk_model_name,where:deleted_at IS NULL"`
     Description string         `json:"description,omitempty" gorm:"type:text"`
     Tags        string         `json:"tags,omitempty" gorm:"type:varchar(255)"`
     VendorID    int            `json:"vendor_id,omitempty" gorm:"index"`