security: avoid raw uploaded filename leakage in service logs (fixes #273)

[Flamki]

Description

Fixes #273.

This PR hardens file_service logging to avoid raw uploaded filename leakage in service logs, while preserving operational signal.

What changed

• chatbot-core/api/services/file_service.py
  • Added _safe_filename_for_log() using existing sanitize_logs utility.
  • Updated warning logs that include filename to use sanitized basename.
  • Replaced INFO log Processing uploaded file: <filename> with metadata-only logging:
    • file size in bytes
    • file extension
• chatbot-core/tests/unit/services/test_file_service.py
  • Added regression test ensuring the INFO log path does not include raw filename values.

Why

Filenames are user-controlled input and can contain sensitive identifiers. This keeps INFO/WARN logs safer by default without changing file-processing behavior or response contracts.

Scope / non-overlap

• Backend logging hardening only.
• No changes to upload validation logic, persistence flow, auth/gatekeeper, or retrieval architecture.

Verification

• pytest tests/unit/services/test_file_service.py -q -> 46 passed
• pylint api/services/file_service.py tests/unit/services/test_file_service.py -> 10.00/10