security(auth): replace LthnHash with bcrypt for password hashing

[Snider]

Parent audit: #38

Finding

LthnHash is not suitable for password hashing. It lacks the computational cost and salting properties required for secure password storage.

Location

• src/Core/Helpers/ — LthnHash implementation
• All password hashing call sites across the codebase

Fix

Replace LthnHash usage with password_hash() (bcrypt) for all password hashing. Implement a migration strategy for existing hashed passwords (rehash on next login).

Acceptance Criteria

• All password hashing uses password_hash() with PASSWORD_BCRYPT or PASSWORD_DEFAULT
• All password verification uses password_verify()
• Migration path for existing passwords (rehash on successful login)
• LthnHash class deprecated or removed
• No plaintext or weakly-hashed passwords remain in the system

[Snider]

Parent: #104