[Scanner] Add SQL Injection patterns

[taladari]

Summary
Add a new scanner to detect SQL injection attempts (classic "UNION SELECT", "DROP TABLE", ";--" etc.).

Why
RAG pipelines sometimes surface database docs/configs. A malicious query might attempt injection via retrieved text.

Acceptance criteria

• New SQLInjectionScanner in rag_firewall/scanners/sql_injection_scanner.py.
• Matches common SQLi patterns.
• Unit tests in tests/test_scanners.py with positive + negative cases.

Difficulty: easy/medium (regex-based)