feat(connector): add Seafile as data source (#12945)

### What problem does this PR solve?
This PR adds **Seafile** as a new data source connector for RAGFlow.

[Seafile](https://www.seafile.com/) is an open-source, self-hosted file
sync and share platform widely used by enterprises, universities, and
organizations that require data sovereignty and privacy. Users who store
documents in Seafile currently have no way to index and search their
content through RAGFlow.

This connector enables RAGFlow users to:
- Connect to self-hosted Seafile servers via API token
- Index documents from personal and shared libraries
- Support incremental polling for updated files
- Seamlessly integrate Seafile-stored documents into their RAG pipelines


### Type of change

- [x] New Feature (non-breaking change which adds functionality)
### Changes included

- `SeaFileConnector` implementing `LoadConnector` and `PollConnector`
interfaces
- Support for API token
- Recursive file traversal across libraries
- Time-based filtering for incremental updates
- Seafile logo (sourced from Simple Icons, CC0)
- Connector configuration and registration

### Testing

- Tested against self-hosted Seafile Community Edition
- Verified authentication (token)
- Verified document ingestion from personal and shared libraries
- Verified incremental polling with time filters

web/src/locales/en.ts

@@ -998,6 +998,16 @@ Example: Virtual Hosted Style`,
         'The base URL of your Moodle instance (e.g., https://moodle.university.edu). Do not include /webservice or /login.',
       moodleTokenTip:
         'Generate a web service token in Moodle: Go to Site administration → Server → Web services → Manage tokens. The user must be enrolled in the courses you want to sync.',
+      seafileDescription:
+        'Connect to your SeaFile server to sync files and documents from your libraries.',
+      seafileUrlTip:
+        'The base URL of your SeaFile server (e.g., https://seafile.example.com). Do not include /api2 or other paths.',
+      seafileTokenTip:
+        'Generate an API token in SeaFile: Go to Settings → API Token → Generate Token. The token provides access to all libraries visible to your account.',
+      seafileIncludeSharedTip:
+        'When enabled, libraries shared with you by other users will also be synced.',
+      seafileBatchSizeTip:
+        'Number of documents to process per batch. Higher values may improve performance but use more memory. Default: 100.',
       jiraDescription:
         'Connect your Jira workspace to sync issues, comments, and attachments.',
       jiraBaseUrlTip: