fix: Resolve all lint warnings and type errors across frontend and backend.

Frontend:
- Memoize pagination object in TransactionsTable to prevent unnecessary re-renders and fix exhaustive-deps warning
- Add optional success and message fields to backup API response types for proper error handling

Backend:
- Add TypedDict for transaction type configuration to improve type safety in generate_sample_db
- Fix unpacking of amount_range with explicit float type hints
- Add explicit type hints for descriptions dictionary and specific_descriptions variable
- Fix sync endpoint return types: get_sync_status returns SyncStatus and sync_now returns SyncResult
- Fix transactions endpoint data type declaration to properly support Union types in PaginatedResponse

All checks now pass:
- Frontend: npm lint and npm build ✓
- Backend: mypy type checking ✓
- Backend: ruff lint on modified files ✓

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

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

frontend/src/components/TransactionsTable.tsx

@@ -1,4 +1,4 @@
-import { useState, useEffect } from "react";
+import { useState, useEffect, useMemo } from "react";
 import { useQuery } from "@tanstack/react-query";
 import {
   useReactTable,
@@ -126,16 +126,20 @@ export default function TransactionsTable() {
   });
 
   const transactions = transactionsResponse?.data || [];
-  const pagination = transactionsResponse
-    ? {
-        page: transactionsResponse.page,
-        total_pages: transactionsResponse.total_pages,
-        per_page: transactionsResponse.per_page,
-        total: transactionsResponse.total,
-        has_next: transactionsResponse.has_next,
-        has_prev: transactionsResponse.has_prev,
-      }
-    : undefined;
+  const pagination = useMemo(
+    () =>
+      transactionsResponse
+        ? {
+            page: transactionsResponse.page,
+            total_pages: transactionsResponse.total_pages,
+            per_page: transactionsResponse.per_page,
+            total: transactionsResponse.total,
+            has_next: transactionsResponse.has_next,
+            has_prev: transactionsResponse.has_prev,
+          }
+        : undefined,
+    [transactionsResponse],
+  );
 
   // Check if search is currently debouncing
   const isSearchLoading = filterState.searchTerm !== debouncedSearchTerm;