fix(import-export): harden file processing and validation

[marynes0111]

Objective

Review import/export functionality and address remaining security or validation concerns.

Scope

• Validate uploaded files
• Review RIS import processing
• Review BibTeX import processing
• Review export behavior
• Improve defensive validation

Acceptance Criteria

• Invalid files are rejected
• Supported formats continue to work
• Import/export tests pass
• No known critical import/export defects remain open