feat: registration gravity from DB config instead of hardcoded

- Add registrationGravity to pricing config (default 50, adjustable via admin)
- Inject PricingService into UserService, read config for all 4 registration paths
- Expose registrationGravity in /member/plans response for frontend
- Login page fetches dynamic value from API, replaces hardcoded '50'
- Update user.vue gravity card copy

backend/src/modules/schemas/pricing.service.ts

@@ -10,6 +10,8 @@ export interface GravityRates {
 }
 
 interface PricingConfig {
+  /** 新用户注册赠送引力值 */
+  registrationGravity: number
   interview: { pricePerSession: number; creditsPerPurchase: number }
   resumeOptimize: { freeLimit: number; pricePerOptimize: number; creditsPerPurchase: number }
   resumeDownload: { pricePerDownload: number; creditsPerPurchase: number }
@@ -21,6 +23,7 @@ interface PricingConfig {
 }
 
 const DEFAULT_PRICING: PricingConfig = {
+  registrationGravity: 50,
   interview: { pricePerSession: 600, creditsPerPurchase: 1 },
   resumeOptimize: { freeLimit: 3, pricePerOptimize: 400, creditsPerPurchase: 1 },
   resumeDownload: { pricePerDownload: 200, creditsPerPurchase: 1 },
@@ -62,6 +65,7 @@ export class PricingService {
 
   private mergeDefaults(value: any): PricingConfig {
     return {
+      registrationGravity: value?.registrationGravity ?? DEFAULT_PRICING.registrationGravity,
       interview: { ...DEFAULT_PRICING.interview, ...value?.interview },
       resumeOptimize: { ...DEFAULT_PRICING.resumeOptimize, ...value?.resumeOptimize },
       resumeDownload: { ...DEFAULT_PRICING.resumeDownload, ...value?.resumeDownload },