feat: device linking, and signing in to the hub with a device key

Stage C. Identity keys are per node, so a browser and a desktop client are two
keys on one account there — and the node refused the second where it accepted
the first. Without this, an account created natively could never be opened in a
browser without an operator code per node, and "a native client must not prevent
web use" would have been dead on arrival.

Device linking (node)
---------------------
`identities` is keyed by `(user_id, pk_ed25519)` instead of `user_id` alone. The
old shape did `INSERT OR REPLACE`, so a second device overwrote the first
silently; SQLite cannot change a primary key in place, so the table is rebuilt.
Existing pins are carried over — verified against a live roster with 10 of them,
nobody re-pairs.

A new device files a request bound by `sha256(code ‖ its own keys)`, and a key
the node **already pinned** countersigns it. The hub cannot: it has stored no
user keys since 2026-08-14, which is what makes this safe to do without an
operator in the loop.

**The code never reaches the node.** It lists this account's pending requests
with their stored hashes; the approver recomputes and keeps the match. A node
offering fabricated keys would have to produce a hash over a code it has never
seen. Nothing rests on a human comparing digits — that ritual was dropped in
12.1 as "correct, unusable as the default" and must not return by the back door.

The design document had the approver look a request up *by* its hash, which is
circular: computing it needs the keys being asked about. Corrected in both.

Revocation marks rather than deletes, because a deleted row is a key the node
would happily pin again — which is the laptop somebody just reported lost. Your
last device cannot be revoked: coming back would need an operator's code.

Hub — the only change in the whole plan
---------------------------------------
`POST /v1/users/auth` signs in with a device Ed25519 key, on the same pattern as
`/v1/nodes/auth`, plus `/v1/users/devices` to register, list and retire. New
`user_devices` table with an Alembic migration, because `create_all()` is not
one.

This is **not** the key directory that was H3, and the tests say so: nothing
reads it but the hub, no group key is ever wrapped for one, and it is a
different key from the per-node identities. What it does cost is metadata — the
hub now knows how many devices an account has and when each last signed in.

Also `client.minimum` / `client.recommended` in `GET /v1/hub/version`: an
installed client meets a newer hub the day the interface ships in a package, and
that is cheap now and awkward to retrofit.

Browser
-------
The `key_changed` refusal becomes `unknown_device` and offers a linking code
instead of telling someone to find their operator. The Members panel lists this
account's devices here, approves one by code, and retires one.

773 tests pass. `e2e.py` gained a step that links a device end to end against
the live deployment — file, list, recompute, countersign, then open the group
with the new keys and no code — and it also gained `recv_type`, because a step
that assumes the next message is its own answer reads an ack left by the step
before.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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