refactor: streamline document upload limits and enhance handling of mentioned documents

- Updated maximum file size limit to 500 MB per file.
- Removed restrictions on the number of files per upload and total upload size.
- Enhanced handling of user-mentioning documents in the knowledge base search middleware.
- Improved document reading and processing logic to accommodate new features and optimizations.

surfsense_backend/app/agents/new_chat/middleware/filesystem.py

@@ -66,6 +66,16 @@ the `<chunk_index>`, identify chunks marked `matched="true"`, then use
 those sections instead of reading the entire file sequentially.
 
 Use `<chunk id='...'>` values as citation IDs in your answers.
+
+## User-Mentioned Documents
+
+When the `ls` output tags a file with `[MENTIONED BY USER — read deeply]`,
+the user **explicitly selected** that document. These files are your highest-
+priority sources:
+1. **Always read them thoroughly** — scan the full `<chunk_index>`, then read
+   all major sections, not just matched chunks.
+2. **Prefer their content** over other search results when answering.
+3. **Cite from them first** whenever applicable.
 """
 
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