feat: add configurable sensitive word detection

[Timik232]

Summary

Adds a new SENSITIVE_WORD entity type that allows detecting user-defined sensitive words via the MASKER_SENSITIVE_WORDS environment variable.

Changes

• New entity type: SENSITIVE_WORD added to EntityType
• Config: MASKER_SENSITIVE_WORDS env var (comma-separated word list)
• Detection: Case-insensitive regex with \b word boundaries
• Overlap handling: Lowest priority — loses to EMAIL, CARD, PHONE, PERSON
• Masking/Redaction: Full support via existing endpoints
• Policy: sensitive_word: mask in default policy

Usage

MASKER_SENSITIVE_WORDS="secret,confidential,password" python -m app.main

curl -X POST /api/v1/detect -d '{"text": "This is a secret document"}'
# Returns: {"entities": [{"type": "SENSITIVE_WORD", "value": "secret", ...}]}

Testing

• Added tests/test_sensitive_words.py with class-based tests
• All existing tests pass without modification
• Tested with Cyrillic words (e.g., MASKER_SENSITIVE_WORDS="СВО")