Expose lightweight file metadata for pre-upload attachment dedupe

[bdart]

Future Outlook attachment dedupe should happen before upload, not only after files are already in chat history.

Add lightweight file-level metadata to uploaded file records and API responses:

• persist size_bytes
• persist content_type
• expose both on FileUploadItem and any history surface used for chat file comparisons

Goal:

• FE can cheaply suppress already-uploaded Outlook attachments using filename + size + content type (+ Outlook source identity when available)
• keep content hashing / semantic diff as a later step, not part of the first implementation