Simplify device authentication: remove wallet tracking from sessions

[Copilot]

PR #915 stored wallet_id, nonce, and signature in devices_sessions. This mixed authentication concerns with wallet management and stored redundant data already tracked elsewhere.

Changes

Database schema - devices_sessions now only tracks when devices authenticate:

CREATE TABLE devices_sessions (
    id SERIAL PRIMARY KEY,
    device_id INT NOT NULL REFERENCES devices(id),
    created_at TIMESTAMP NOT NULL DEFAULT current_timestamp
);

Auth guard - Removed wallet lifecycle management:

• VerifiedAuth no longer includes wallet_id or wallet_identifier
• Auth guard verifies request, stores session timestamp, returns device + address
• Wallet operations moved to business logic layer

Referral endpoints - Wallet lookups now where they're needed:

// Before: used pre-computed wallet_identifier from auth
create_username(&request.auth.wallet_identifier, ...)

// After: compute when needed
let wallet_identifier = WalletId::Multicoin(request.auth.address.clone()).id();
create_username(&wallet_identifier, ...)

Net: -29 LOC, cleaner separation of concerns.

Original prompt

#915

we added device authentication now, see how we pass headers, review pr, simplify, only need to store devices and date as created at in db

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