feat: configurable default KDF type for new user registrations

[dnplkndll]

Summary

Adds CLIENT_KDF_TYPE, CLIENT_KDF_ITERATIONS, CLIENT_KDF_MEMORY, and CLIENT_KDF_PARALLELISM environment variables to allow server admins to configure the default KDF for new user registrations.

Closes #6982

Motivation

The default KDF is currently hardcoded to PBKDF2 with 600,000 iterations. Argon2id is memory-hard and significantly more resistant to GPU-based brute-force attacks, but admins have no way to set it as the default without modifying source code. This is particularly important for self-hosted family/team instances where users are unlikely to change their KDF settings manually.

Changes

• src/config.rs — Add 4 config entries with input validation matching existing Argon2id parameter validation
• src/db/models/user.rs — Replace hardcoded CLIENT_KDF_TYPE_DEFAULT / CLIENT_KDF_ITER_DEFAULT constants with config-backed functions. Automatically uses sensible Argon2id defaults (3 iterations, 64MB, 4 parallelism) when CLIENT_KDF_TYPE=1
• src/api/core/accounts.rs — Use config values for prelogin response when email is not found (previously hardcoded to PBKDF2)
• .env.template — Document new variables

Backwards Compatibility

• Default values produce identical behavior to current hardcoded constants
• Existing users are not affected — only new account creation and the prelogin response for unknown emails

Testing

Tested on a self-hosted instance with CLIENT_KDF_TYPE=1:

• Prelogin endpoint returns {"kdf": 1, "kdfIterations": 3, "kdfMemory": 64, "kdfParallelism": 4} for unknown emails
• New account registration creates accounts with Argon2id KDF
• Existing accounts retain their current KDF settings
• Validation rejects invalid parameters (e.g., memory < 15MB, parallelism > 16)

Example

CLIENT_KDF_TYPE=1

That's it — sensible Argon2id defaults are applied automatically.