feat(correlation/attribution): substrate + idle handler (Phase 1)

v0 Phase 1 of ATTRIBUTION-ENGINE.md:

* AttributionStateRow SQLModel keyed on (identity_uuid, primitive)
  per ANTI direction — re-keying state rows when the v1 clusterer
  merges attackers is the migration debt v0 should not bake in.
  ATTRIBUTION-ENGINE.md updated with the deviation note.
* AttributionMixin: ensure_stub_identity_for_attacker, idempotent
  upsert_attribution_state, get_attribution_state[_for_identity],
  list_multi_actor_identities (the Phase 5 correlator's read).
* attribution.profile.{state_changed,multi_actor_suspected} bus
  topics + builder; wiki Service-Bus.md updated separately.
* attribution_worker.py: subscribes to attacker.observation.>,
  ensures stub identity per event, logs and continues. No merger,
  no state writes, no derived events — Phase 4 wires those.
* attribution/{aggregate.py,_thresholds.py} skeletons: Phase 2
  fills _aggregate_categorical, Phase 3 adds numeric+hash+dispatcher.

decnet/web/db/sqlmodel_repo/__init__.py

@@ -35,6 +35,7 @@ from decnet.web.db.sqlmodel_repo._helpers import (  # noqa: F401  (re-exported f
 )
 from decnet.web.db.sqlmodel_repo.attacker_intel import AttackerIntelMixin
 from decnet.web.db.sqlmodel_repo.attackers import AttackersMixin
+from decnet.web.db.sqlmodel_repo.attribution import AttributionMixin
 from decnet.web.db.sqlmodel_repo.auth import AuthMixin
 from decnet.web.db.sqlmodel_repo.bounties import BountiesMixin
 from decnet.web.db.sqlmodel_repo.campaigns import CampaignsMixin
@@ -58,6 +59,7 @@ from decnet.web.db.sqlmodel_repo.webhooks import WebhooksMixin
 class SQLModelRepository(
     AttackerIntelMixin,
     AttackersMixin,
+    AttributionMixin,
     AuthMixin,
     BountiesMixin,
     CampaignsMixin,