fix: Validate and sanitize BookDB response data (#255)

[deucebucket]

Summary

• Adds _sanitize_api_response() helper in library_manager/providers/bookdb.py that strips null bytes, control characters, HTML tags, and truncates oversized string fields from API responses
• Applied to search_bookdb() (top-level response + best_book), identify_audio_with_bookdb() result dict, and lookup_fingerprint() in fingerprint.py
• Prevents path traversal, XSS, and oversized field injection from corrupted or tampered BookDB/Skaldleita responses

Closes #255

Test plan

• Verify python test-env/test-naming-issues.py passes (290/290)
• Verify ruff check . --select=F821 passes
• Confirm sanitization logs appear when BookDB returns fields with control chars or HTML
• Confirm normal BookDB responses pass through unchanged