Remove --decrypt-db migration tool in v0.18.0

[mostronatorcoder]

Context

As part of #642, we removed full database encryption and kept --decrypt-db as a one-time migration tool so operators with encrypted databases can transition to plaintext.

This migration path was introduced in PR #644 and preserved in PR #647.

Version strategy

• v0.17.0 (transition release): encryption removed from normal flow, --decrypt-db available for migration
• v0.18.0 (this issue): remove --decrypt-db and all remaining encryption dependencies

What to remove in v0.18.0

• --decrypt-db CLI flag (src/cli.rs)
• decrypt_database() function (src/db.rs)
• get_admin_password() function (src/db.rs)
• validate_db_password RPC stub (src/rpc/service.rs) — currently returns always-success
• argon2 + secrecy dependencies (if no longer used elsewhere)
• CryptoUtils usage in decrypt_database (verify if still needed elsewhere)

Why wait

Operators running encrypted databases need at least one minor release (v0.17.0) with --decrypt-db available before we remove the migration path entirely in v0.18.0.