fix(blockchain): read token decimals from contract for deposit detection

Previously decimals were hardcoded to 18, causing incorrect amount parsing
for USDT which uses 6 decimals. Now reads decimals() from ERC20 contract
when processing deposits.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/services/blockchain-service/src/application/services/deposit-detection.service.ts

@@ -143,6 +143,9 @@ export class DepositDetectionService implements OnModuleInit {
       return;
     }
 
+    // 获取代币的实际 decimals（USDT 通常是 6 位，而不是 18 位）
+    const tokenDecimals = await this.evmProvider.getTokenDecimals(chainType, event.tokenContract);
+
     // 创建充值记录 - 用户地址
     const deposit = DepositTransaction.create({
       chainType,
@@ -150,7 +153,7 @@ export class DepositDetectionService implements OnModuleInit {
       fromAddress: EvmAddress.fromUnchecked(event.from),
       toAddress: EvmAddress.fromUnchecked(event.to),
       tokenContract: EvmAddress.fromUnchecked(event.tokenContract),
-      amount: TokenAmount.fromRaw(event.value, 18),
+      amount: TokenAmount.fromRaw(event.value, tokenDecimals),
       blockNumber: BlockNumber.create(event.blockNumber),
       blockTimestamp: event.blockTimestamp,
       logIndex: event.logIndex,

backend/services/blockchain-service/src/infrastructure/blockchain/evm-provider.adapter.ts

@@ -137,6 +137,16 @@ export class EvmProviderAdapter {
     return TokenAmount.fromRaw(balance, Number(decimals));
   }
 
+  /**
+   * 查询 ERC20 代币的 decimals
+   */
+  async getTokenDecimals(chainType: ChainType, tokenContract: string): Promise<number> {
+    const provider = this.getProvider(chainType);
+    const contract = new Contract(tokenContract, ERC20_BALANCE_ABI, provider);
+    const decimals = await contract.decimals();
+    return Number(decimals);
+  }
+
   /**
    * 查询原生代币余额
    */