refactor(deploy): rename system user lamassu → bitspire

System-level half of the lamassu → bitspire rebrand the rest of dev
already did at the path / service / package layers. Touches user/group
declarations, every systemd `User=` block, the udev rules filename, all
chown calls in flake.nix + live.nix, the displayManager autoLogin user,
the trusted-users nix entry, provision-atm.sh's ATM_USER, plus README +
CLAUDE.md doc references.

In-place migration for the Sintra dev unit (which auto-pulls dev at
04:00) lives in `system.activationScripts.bitspire-user-migration` and:
- copies `/home/lamassu/.ssh/authorized_keys` → `/home/bitspire/` once,
  so SSH access survives the rename
- recursively chowns `/var/lib/bitspire` to the new bitspire UID on
  every boot — cheap no-op once done, but covers the case where the
  data dir was written by the now-removed lamassu UID
- leaves `/home/lamassu/` in place as evidence; operator can `rm -rf`
  after confirming bitspire login works

Recovery path if the migration breaks SSH access: root key is still in
configuration.nix:142-144 (padreug@gizmo), so ssh root@<host> works.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -189,7 +189,7 @@ UP Board enumerates its eMMC controller via ACPI, not PCI. `upboard.nix` force-l
 
 ## Security priorities
 
-1. **Private keys** — Never log nsec. The ATM's `VITE_ATM_PRIVATE_KEY` lives in `/var/lib/bitspire/.env` with mode 0600, owned by `lamassu:lamassu`.
+1. **Private keys** — Never log nsec. The ATM's `VITE_ATM_PRIVATE_KEY` lives in `/var/lib/bitspire/.env` with mode 0600, owned by `bitspire:bitspire`.
 2. **Payments** — Validate the bolt11 amount on cash-out before exposing the QR. Decode `payment_hash` from the bolt11 (cheap, avoids a roundtrip) and use it as the `subscribe_payments` filter.
 3. **Replay** — LNURL-withdraw links use `uses:1` and are deleted on session abort.
 4. **Encryption** — All RPC content is NIP-44 v2. NIP-04 is forbidden.