feat: add Postgres MCP integration for database testing

- Add postgres-mcp service to docker-compose.yml (SSE mode on port 8000)
- Add .mcp.json.example with SSE configuration template
- Add .gitignore entries for .claude/settings.local.json and .mcp.json
- Add MCP_EXAMPLES.md with query examples for testing scraping results
- Add analysis scripts: analyze-category-nulls.ts, check-product-details.ts,
  inspect-api-response.ts

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

.mcp.json.example

@@ -0,0 +1,8 @@
+{
+  "mcpServers": {
+    "postgres-supermarket": {
+      "type": "sse",
+      "url": "http://localhost:8000/sse"
+    }
+  }
+}