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>

main.py

@@ -1,6 +1,6 @@
-"""RMI Backend — root entry point.
+"""RMI Backend - root entry point.
 
-Minimal — delegates everything to app.factory.create_app().
+Minimal - delegates everything to app.factory.create_app().
 
 Per v4.0 §T01 + modern factory pattern:
 - main.py is the entry point ONLY