fix: update somark.tech to somark.cn/somark.ai with purchase URLs (#16892)

## Summary

Update all `somark.tech` references to `somark.cn` (default for China) /
`somark.ai` (for overseas including Taiwan, China; Hong Kong, China;
Macau, China). Users fill in `base_url` manually — default is
`somark.cn`.

### Changes

| File | Change |
|------|--------|
| `constants.py` | Default → `somark.cn` |
| `somark_parser.py` | SAAS_BASE_URL + fallback → `somark.cn` |
| `ocr_model.py` | Default → `somark.cn` |
| `pdf_parser_common.go` | Go default → `somark.cn` |
| `llm.ts` | API key URL → `somark.cn` |
| `en.ts` | Base URL descriptions + purchase URLs |
| `zh.ts` | Base URL descriptions + purchase URLs |

### Purchase URLs
- China: `https://somark.cn/workbench/purchase`
- Overseas: `https://somark.ai/studio/purchase`

---------

Co-authored-by: justinychuang <huangyicheng@soulcode.cn>
Co-authored-by: Jin Hai <haijin.chn@gmail.com>

deepdoc/parser/somark_parser.py

@@ -121,7 +121,7 @@ class SoMarkParser(RAGFlowPdfParser):
 
     # /usage quota check only works in SaaS; private deployments fall back
     # to a generic HEAD health check.
-    SAAS_BASE_URL = "https://somark.tech/api/v1"
+    SAAS_BASE_URL = "https://somark.cn/api/v1"
     USAGE_REQUEST_TIMEOUT = 10  # /usage request timeout
 
     # SoMark error codes
@@ -749,7 +749,7 @@ class SoMarkParser(RAGFlowPdfParser):
 
 if __name__ == "__main__":
     parser = SoMarkParser(
-        base_url=os.environ.get("SOMARK_BASE_URL", "https://somark.tech/api/v1"),
+        base_url=os.environ.get("SOMARK_BASE_URL", "https://somark.cn/api/v1"),
         api_key=os.environ.get("SOMARK_API_KEY", ""),
     )
     ok, reason = parser.check_installation()