style(rmi-backend): complete lint cleanup — 1175→0 ruff errors

- Fix 71 invalid-syntax files (class-body newline-broken assignments)
- Add from/None chain to 307 B904 raise-without-from sites
- Add B008 ignore to ruff.toml (already in pyproject.toml)
- Noqa F401 on __init__.py re-exports (137 sites)
- Noqa E402 on deferred imports (63 sites)
- Bulk-add stdlib/FastAPI/project imports for F821 (127 sites)
- Replace ×→x, –→-, …→... in docstrings (4093 chars)
- Manual refactor of 5 SIM103/SIM116 patterns

Tests: 791 passed (66 deselected due to pre-existing Redis issues in test_rag.py)
Co-authored-by: opencode <opencode@rugmunch.io>

databus_warm_cron.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python3
 """
-DataBus Cache Warmer — called by cron every 5 minutes.
+DataBus Cache Warmer - called by cron every 5 minutes.
 Seeds top-10 most-missed chains if not present, then starts/verifies
 the background warm loop.
 """