Premium: Key-Based Login Command (cepha login <key>)

[sultanAalyami]

Description

Implement cepha login command using Matryoshka key fingerprint verification.

Tasks

• Create LoginCommand.cs with key verification via MatryoshkaKeyVault.VerifyKeyProofAsync()
• Create PremiumSession record (key hash, features list, expiry)
• Extend auth.db schema for key-based sessions
• Add cepha logout command to clear key session
• Update InteractiveMenu to show login/logout state
• Constant-time key comparison (CryptographicOperations.FixedTimeEquals)

Context

• Existing auth: WebAuthn/PassKey (StoreAuth.cs) remains primary
• Key-based auth is secondary/alternative path for premium activation
• Key verification uses Matryoshka key vault (EntityCrypt.EFCore)

Dependencies

None (entry point)

Migrated from: WasmMvcRuntime #41