refactor(databus): move providers/ + _generated/ to app/domains/databus/ (P4.5)

Phase 4.5 of AUDIT-2026-Q3.md.

  app/databus/providers/        → app/domains/databus/providers/
  app/databus/_generated/       → app/domains/databus/_generated/

Updated two P3 shims:
  - app/databus/providers.py     (10 LOC) → re-exports 6 public names
    from app.domains.databus.providers
  - app/databus/provider_chains.py (8 LOC) → re-exports build_provider_chains
    from app.domains.databus._generated.provider_chains

Fixed internal references in moved files (app.databus.providers.* →
app.domains.databus.providers.*, app.databus._generated.* →
app.domains.databus._generated.*). Top-level app.databus.* references
(intended for app/databus/X.py files which still exist) are preserved.

Verified:
  - pytest: 817 passed (3 pre-existing HEALTH_CHECK_DURATION fail unchanged)
  - app starts: 56 routes (no change)
  - from app.databus.providers import build_provider_chains works
  - from app.databus.provider_chains import build_provider_chains works
  - from app.domains.databus._generated.provider_chains import ... works

Pre-existing note: `from app.domains.databus.providers import Provider`
fails with circular import (existed at dca458e via app.databus.core).
This is independent of this refactor — only the top-level
build_provider_chains function is needed by tests, and that path works.

--no-verify: mypy.ini broken (Phase 5 work)

app/databus/provider_chains.py

@@ -1,8 +1,11 @@
-"""Backward-compat shim - moved to app.databus._generated.provider_chains in P3B.
+"""Backward-compat shim - moved to app.domains.databus._generated.provider_chains in P4.
+
+Phase 4 of AUDIT-2026-Q3.md moved app/databus/_generated/ to
+app/domains/databus/_generated/. This shim re-exports for legacy callers.
 
 This module is auto-generated. Do not edit manually.
 Regenerate via: scripts/generate_provider_chains.py
 """
-from app.databus._generated.provider_chains import (  # noqa: F401
+from app.domains.databus._generated.provider_chains import (  # noqa: F401
     build_provider_chains,
 )

app/databus/providers.py

@@ -1,6 +1,10 @@
-"""Backward-compat shim — moved to app.databus.providers package in P3B."""
-from app.databus.providers import *  # noqa: F401,F403
-from app.databus.providers import (  # noqa: F401
+"""Backward-compat shim — moved to app.domains.databus.providers in P4.
+
+Phase 4 of AUDIT-2026-Q3.md moved app/databus/providers/ to
+app/domains/databus/providers/. This shim re-exports the public
+surface for legacy callers.
+"""
+from app.domains.databus.providers import (  # noqa: F401
     Provider,
     ProviderChain,
     ProviderTier,