Was: unsalted SHA-256 read from [system] arm_pin_hash.
Now: PBKDF2-SHA256 600k iterations read from [security] pin_hash,
matching the web arm/disarm path and the alerts/pin module.
Also drops the redundant pin re-hash on the arm_state_log audit row
(a fresh PBKDF2 salt made the column valueless for traceability).
Part of issue #2 PIN hashing unification.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>