feat: persist sourceFile in MCP annotation store

[FreeWall]

Summary

The browser toolbar already captures sourceFile and sends it when syncing annotations, but the MCP server dropped it on save.
This adds sourceFile persistence so agents can see the source file path.

• Add sourceFile to the MCP Annotation type
• Store it in SQLite via a new source_file column (schema + startup migration)
• Map the field on insert and read in rowToAnnotation
• Expose sourceFile in MCP tool responses via mapAnnotationForMcp
• Update docs & schema

Related to #54, 882a827

[vercel]

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