Support file uploads on document update and enable admin document creation

[iqra1503]

Motivation

• Document edits that included uploaded replacement files were not being saved because the update endpoint only accepted JSON payloads.
• Admins lacked the ability to create documents from the admin panel using the same upload/summary flow available to regular users.

Description

• Update the backend PUT /documents/{document_id} handler to accept multipart/form-data (now async def update_document(...) accepts file, title, description, and summary) and extract uploaded file text with extract_text_from_upload while preserving the existing JSON update flow via DocumentUpdate validation.
• When processing multipart updates the backend sets source_type, file_name, and file_type, and regenerates summary_embedding when summary is updated.
• Change the frontend updateDocumentApi to send FormData when a file is present so file-based updates match the backend contract.
• Extend the shared DocumentForm to allow file uploads in edit mode (and make the summary preview call a no-op when not provided), and wire the admin dashboard to use the same form so admins can create and edit documents and preview summaries.

Testing

• Built the frontend with npm run build and the build completed successfully.
• Compiled backend modules with python -m compileall backend/app and compilation succeeded without errors.
• Manually exercised the frontend dev server and captured a screenshot to validate the admin form UI changes (playwright script run succeeded).

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Codex Task

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