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.

cassette_transport.py

@@ -141,8 +141,10 @@ def _state_d_tag(atm_pubkey_hex: str) -> str:
 
 def build_state_d_tags_for_machines(machines: list[Machine]) -> list[str]:
     """Bootstrap-consumer subscription filter helper: returns the full
-    `#d=[...]` list for all known ATMs an operator subscribes to."""
-    return [_state_d_tag(_atm_hex_pubkey(m)) for m in machines]
+    `#d=[...]` list for all known PAIRED ATMs an operator subscribes to.
+    Unpaired machines (machine_npub is None — nullable since #29/m011) have no
+    state-beacon d-tag yet, so skip them rather than crash `_atm_hex_pubkey`."""
+    return [_state_d_tag(_atm_hex_pubkey(m)) for m in machines if m.machine_npub]
 
 
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