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>

app/api/v1/admin/__init__.py

@@ -1,4 +1,4 @@
-"""Admin routes — admin role required.
+"""Admin routes - admin role required.
 
 Target: user management, system config, ops, bulletin moderation.
 """

app/api/v1/admin/alerts_webhook.py

@@ -1,4 +1,4 @@
-"""Admin alerts webhook — /api/v1/admin/alerts/webhook.
+"""Admin alerts webhook - /api/v1/admin/alerts/webhook.
 
 Stub endpoint for receiving alert webhooks from external sources
 (monitoring, observability platforms).
@@ -28,5 +28,5 @@ async def receive_alert_webhook(payload: AlertWebhookPayload) -> dict[str, Any]:
     """Receive an alert webhook from external monitoring."""
     raise HTTPException(
         status_code=501,
-        detail="Alert webhook ingestion not yet implemented — pending T08 GlitchTip wiring",
+        detail="Alert webhook ingestion not yet implemented - pending T08 GlitchTip wiring",
     )