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>

scripts/migrate_redis_to_pgvector_v2.py

@@ -102,7 +102,7 @@ async def insert_with_retry(client: httpx.AsyncClient, rows: list, batch_num: in
             if resp.status_code in (200, 201):
                 return len(rows)
             elif resp.status_code == 500 and "timeout" in (resp.text or "").lower():
-                # Statement timeout — split into smaller batches
+                # Statement timeout - split into smaller batches
                 logger.warning(f"Batch {batch_num}: statement timeout (attempt {attempt + 1}), splitting")
                 if len(rows) > 10:
                     # Split into two halves