feat!: replace flat master-key vault encryption with Argon2id KEK/DEK model

[manojbajaj95]

Motivation

The previous vault used Fernet (AES-128-CBC + HMAC-SHA256) with a master key loaded directly from env, file, or keyring — a flat, single-key model where the master secret encrypted every credential blob directly. This design has two critical weaknesses:

1. No key rotation without re-encrypting everything. Because the master key is the encryption key, rotating it means decrypting and re-encrypting every stored credential.
2. Single-point-of-failure secret. If an attacker recovers the master key, every credential in every collection is immediately readable.

Design

Envelope encryption — the same model used by AWS Secrets Manager, HashiCorp Vault, and GCP KMS:

master_secret (env → file → keyring → auto-generate)
    └─▶ Argon2id(master_secret, salt) ──▶ KEK  [never stored]
              └─▶ AES-256-GCM.decrypt(KEK, wrapped_dek) ──▶ DEK  [in memory only]
                        └─▶ AES-256-GCM(DEK, plaintext) ──▶ encrypted blob  [in KV store]

Why a DEK?
The random 256-bit DEK is what actually encrypts credential blobs. The master secret only ever touches the DEK (wrapped + stored in __vault_meta__:__dek__). Key rotation now means re-wrapping one DEK record, not re-encrypting every credential.

Why Argon2id for all master secrets?
Previously there were two code paths — passphrase (KDF) vs raw key (direct). Maintaining two paths is error-prone and confusing. Argon2id on a high-entropy random key is redundant but safe — the cost is a ~100ms KDF on startup, which is acceptable. A single path eliminates the distinction entirely.

Why store the wrapped DEK in the KV store?
The DEK record lives at __vault_meta__:__dek__ in the same KV backend as credentials. This means the vault works with any KV backend (local DiskStore, remote DynamoDB, Redis) without a sidecar file — important for serverless and distributed deployments. The wrapped DEK is protected by AES-256-GCM with the KEK; it is useless without the master secret.

Why AES-256-GCM over Fernet?
AES-256-GCM is an AEAD primitive (authenticated encryption with additional data) with a 256-bit key. It is the current standard for symmetric encryption and is directly available in cryptography.hazmat. Fernet uses AES-128-CBC + HMAC-SHA256 (two primitives, 128-bit key) and is a higher-level convenience wrapper not intended for custom key hierarchies.

Changes

• MasterSecretResolver: unified AUTHSOME_MASTER_KEY resolution (env → file → keyring → auto-generate); passphrase and raw key both go through Argon2id — no separate code paths
• DekManager: generates a random 256-bit DEK, wraps it with an Argon2id-derived KEK (AES-256-GCM), and stores the wrapped record in the KV store under __vault_meta__:__dek__
• AesGcmEncryptionWrapper: BaseEncryptionWrapper subclass using AES-256-GCM per-value encryption; replaces FernetEncryptionWrapper
• Vault: simplified — takes an already-encrypted AsyncKeyValue; close() removed (caller manages store lifecycle)
• EncryptionConfig removed from ServerConfig and auth.models.__all__
• Health route updated to report crypto_source / crypto_mode from Vault properties
• Tests rewritten with fixtures and SimpleStore (in-memory); DiskStore removed from test suite

Breaking changes

Existing Fernet-encrypted vaults cannot be read after this change. Users will need to re-import credentials (authsome login again for each provider).

Test plan

• uv run pytest — all 321 tests pass
• uv run ruff check src/ tests/ — clean
• uv run ty check src/ — clean
• Manual: authsome init && authsome login github on a fresh ~/.authsome
• Manual: restart daemon — confirm DEK survives reload (same __vault_meta__:__dek__ record, same plaintext retrieved)
• Manual: GET /health returns "encryption_source": "aes-256-gcm" and "encryption_backend": "Argon2id"

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