feat(auth): support Argon2id alongside legacy scrypt

[thecompez]

Summary

• Add configurable Argon2id password hashing backed by libsodium, while retaining verification of all existing scrypt$v1$ hashes.
• Allow algorithm selection during installation and via openproof config hashing, with explicit warnings for compatibility, rehashing, downgrade, pepper preservation, and binary rollback.
• Use the stored hash format to select the verifier, regardless of the currently configured algorithm. Existing passwords and accounts are never bulk-rewritten.
• Opportunistically rehash only after successful complete authentication, with a conditional PostgreSQL update that cannot overwrite a concurrent password change.
• Require a functioning Argon2id backend when Argon2id is selected, and include the runtime dependency in release packages.
• Fix fresh-install migration ordering by deferring three existing tenant-specific client seeds until their organization exists, without changing migration checksums.

Validation

• 485/485 GitHub CI tests passed, including disposable PostgreSQL integration tests for both KDF migration and tenant-specific migrations.
• Build, shell/static checks, GitGuardian, and release packaging succeeded.
• Live OpenProof site stayed healthy throughout development; no user password hashes or production database records were changed.

Operational notes

• Merged to main as ac4e256.
• Not deployed to production yet: openproof-canonical.service requires an authorized service restart and release activation.
• Once any Argon2id hashes exist, rollback must retain dual-algorithm verification. Preserve the existing password pepper.