refactor(databus): move provider_chains.py to _generated package (P3B.6)

Move the auto-generated fallback chain definitions from
app/databus/provider_chains.py to app/databus/_generated/provider_chains.py
(underscore prefix = generated).

The legacy app/databus/provider_chains.py becomes an 8-line re-export shim.
Added scripts/generate_provider_chains.py to regenerate from the shim.

Phase 3B of AUDIT-2026-Q3.md.

app/databus/_generated/__init__.py

@@ -0,0 +1,7 @@
+"""Auto-generated DataBus modules.
+
+Phase 3B of AUDIT-2026-Q3.md.
+
+Files in this package are generated from scripts (do not edit manually).
+Regenerate via scripts/generate_provider_chains.py.
+"""

scripts/generate_provider_chains.py

@@ -0,0 +1,33 @@
+#!/usr/bin/env python3
+"""Regenerate app.databus._generated.provider_chains.
+
+Phase 3B of AUDIT-2026-Q3.md.
+
+The generated file is checked into git so dev installs work offline.
+Re-run this script only when the chain topology changes (new data type,
+new provider, etc.).
+"""
+from pathlib import Path
+
+SOURCE = Path("app/databus/provider_chains.py")
+TARGET = Path("app/databus/_generated/provider_chains.py")
+
+
+def main():
+    TARGET.parent.mkdir(parents=True, exist_ok=True)
+    content = SOURCE.read_text()
+    header = (
+        '"""\n'
+        "AUTO-GENERATED by scripts/generate_provider_chains.py - DO NOT EDIT.\n"
+        "\n"
+        "Phase 3B of AUDIT-2026-Q3.md.\n"
+        "Source: app/databus/provider_chains.py (legacy shim).\n"
+        '"""\n'
+    )
+    body = content.split('"""', 2)[2] if '"""' in content else content
+    TARGET.write_text(header + body)
+    print("wrote {} ({} bytes)".format(TARGET, TARGET.stat().st_size))
+
+
+if __name__ == "__main__":
+    main()