fix: complete remaining 8 optimization items

- HTTP connection pooling: UpstreamClient & WebsiteClient reuse httpx.Client
- Deduplicate decimal_string into shared app/utils/number.py
- Split scheduler transaction: snapshot write → webhook/website sync in separate sessions
- Remove hardcoded 170.106.100.210 migration from database.py
- Reset consecutive_failures on upstream update
- Healthcheck: install curl, replace python -c with curl -f
- Add .dockerignore to reduce build context
- Frontend: add axios-retry with exponential backoff (5xx/network errors only)

frontend/src/api/index.ts

@@ -1,4 +1,5 @@
 import axios from 'axios'
+import axiosRetry from 'axios-retry'
 import router from '@/router'
 import { authStorageKeys } from '@/authStorage'
 
@@ -7,6 +8,19 @@ export const api = axios.create({
   timeout: 30000,
 })
 
+axiosRetry(api, {
+  retries: 3,
+  retryDelay: axiosRetry.exponentialDelay,
+  retryCondition: (err) => {
+    // Retry on network errors or 5xx, but never on 401/403/404/4xx
+    if (!err.response) return true
+    return err.response.status >= 500 && err.response.status < 600
+  },
+  onRetry: (_retryCount, _err, _requestConfig) => {
+    // no-op — could log in dev
+  },
+})
+
 api.interceptors.response.use(
   (r) => r,
   (err) => {