fix: use Argon2id for password hashing

[cloudcarver]

Summary

• replace fast SHA-256 password storage with versioned Argon2id hashes and random salts
• verify encoded hashes in constant time with bounded parameters
• retain legacy SHA-256 verification and upgrade legacy records after successful login
• use a compare-and-swap database update during automatic upgrades
• add malformed, wrong-password, randomization, compatibility, and upgrade tests

Validation

• go test ./...
• go test -race ./pkg/utils ./pkg/service
• go vet ./...

Closes #40