fix: guard every machine_npub deref against unpaired machines (None)

machine_npub became nullable in #29/m011 (register-unpaired flow), but
several consumers still assumed it's non-None and crashed
`normalize_public_key(None)` with `AttributeError: 'NoneType' object has no
attribute 'startswith'`. On the demo (which had an unpaired machine) this
broke the platform-fee update (500) and spammed the cassette consumer with
errors every 2s. The #29 create/pair paths were guarded; these were missed:

- views_api `api_update_super_config`: the "republish fee to every active
  machine" loop → skip unpaired (they get their config at pairing).
- cassette_transport `build_state_d_tags_for_machines`: skip unpaired (no
  state-beacon d-tag yet) — the cassette-consumer loop crash.
- crud `get_machine_by_atm_pubkey_hex`: its `except (ValueError,
  AssertionError)` didn't catch the AttributeError; skip unpaired before
  normalize — the cassette event-handler crash.
- bitspire `assert_nostr_attribution`: reject (SettlementAttributionError) an
  unpaired machine instead of crashing the payment listener.
- views_api cassettes/publish endpoint: 400 (not paired) instead of crashing
  publish_to_atm.

Verified on the dev stack: with an unpaired active machine present, the
cassette consumer registers (skipping it) and runs clean — no AttributeError.

bitspire.py

@@ -126,6 +126,14 @@ def assert_nostr_attribution(machine: Machine, extra: dict) -> None:
             "missing nostr_sender_pubkey on Payment.extra — invoice was not "
             "issued through the nostr-transport path"
         )
+    if not machine.machine_npub:
+        # Unpaired machine (machine_npub None — nullable since #29/m011). It has
+        # no identity to attribute a settlement to; reject cleanly rather than
+        # let normalize_public_key(None) raise an uncaught AttributeError.
+        raise SettlementAttributionError(
+            f"machine {machine.id} is unpaired (no machine_npub); "
+            "a settlement cannot be attributed to it"
+        )
     from lnbits.utils.nostr import normalize_public_key
 
     try: