docs(readme): revert starchart.cc → star-history.com

Revert a44d4a7. star-history.com is back up and the casing-redirect
issue can be worked around case-by-case if it recurs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: brycewang-stanford

README.md

@@ -1393,7 +1393,7 @@ preferred citation and `sp.citation()` will be updated to return it.
 
 ## Star History
 
-[![Stargazers over time](https://starchart.cc/brycewang-stanford/StatsPAI.svg)](https://starchart.cc/brycewang-stanford/StatsPAI)
+[![Star History Chart](https://api.star-history.com/svg?repos=brycewang-stanford/StatsPAI&type=Date)](https://star-history.com/#brycewang-stanford/StatsPAI&Date)
 
 ## License
 

README_CN.md

@@ -609,7 +609,7 @@ sp.__citation__                 # 与 sp.citation("bibtex") 等价
 
 ## Star 历史
 
-[![Stargazers over time](https://starchart.cc/brycewang-stanford/StatsPAI.svg)](https://starchart.cc/brycewang-stanford/StatsPAI)
+[![Star History Chart](https://api.star-history.com/svg?repos=brycewang-stanford/StatsPAI&type=Date)](https://star-history.com/#brycewang-stanford/StatsPAI&Date)
 
 ## 许可证