Refactor SSN encryption: per-record salt and deterministic HMAC lookup

[coderabbitai]

Background

This issue tracks the planned refactoring of SSN encryption, identified during review of PR #80 (comment: #80 (comment)).

Requested by: @codebyNorthsteep

Problem

The current implementation uses Encryptors.text(encryptionKey, salt) (Spring Security) with a single static salt stored as an environment variable. Because Encryptors.text() generates a fresh random 16-byte IV per encryption call:

1. Uniqueness checks are unreliable: staffRepository.existsBySocialSecurityNumber(encryptionService.encrypt(ssn)) will almost never match an existing row because each encryption of the same SSN yields a different ciphertext.
2. Update comparison is broken: Comparing the stored encrypted SSN against a newly encrypted value will always differ.

The current workaround (findAll() + decrypt-and-compare) is O(n) and will degrade as the dataset grows. It is marked with TODO comments in EmploymentFormService.

Planned Solution (as agreed in PR #80)

1. Per-record random salt: Generate a unique salt for each SSN at encryption time and store it alongside the encrypted value in the database, so decryption remains possible without a shared static salt.
2. Deterministic HMAC lookup column: Add a ssn_lookup column to the Staff and EmploymentForm entities containing a keyed HMAC-SHA256 of the plaintext SSN (using a separate, dedicated HMAC key from env vars). This restores O(1) existence checks.
3. Service updates: Refactor EmploymentFormService.validateSsnNotExists() and updateFormBeforeApproval() to query/compare using the HMAC lookup value instead of the full encrypted ciphertext.
4. Key rotation strategy: Document how the HMAC key and encryption key can be rotated.

Acceptance Criteria

• Per-record salt is generated and persisted alongside each encrypted SSN
• A ssn_lookup (HMAC-SHA256) column is added and populated on create/update
• validateSsnNotExists() uses existsBySsnLookup(computeHmac(ssn))
• updateFormBeforeApproval() compares HMAC values for change detection
• Static env-var salt is no longer used as the sole salt for all encryptions
• Unit tests updated to reflect the new approach (no more stubbing encrypt() for uniqueness checks)
• Key rotation strategy is documented