Split into a FastAPI backend and a React frontend, add case state and SSO

The single-file stdlib server became the limit: no way to track what had been
done about an alert, no accounts, and a UI that had to be hand-rolled in
template strings. This restructures it into something deployable.

Backend (FastAPI)
- app/ holds config, database, auth, delivery and the routers; triagelib keeps
  the triage engine unchanged, so the validated screening and runbook logic is
  untouched.
- Cases persist per alert fingerprint with a status workflow (investigating,
  customer contacted, escalated to Infra, waiting, remediated, resolved, won't
  fix, false positive), an assignee, notes and an append-only history. An alert
  that stops and re-fires lands back on the same case and counts as a reopen.
- Suppression rules move from a JSON file into the database.

Auth
- Signed session cookies over PBKDF2 local accounts, plus an OIDC flow ready for
  Authentik: users are created on first login and admin follows a group claim.
  Local login can be switched off entirely once SSO is live.

Zendesk and Jira
- Delivery is now implemented, behind three gates: the integration must be
  configured, its feature flag on, and CX_FEATURE_SEND_ENABLED on. A demo
  instance leaves the last off and cannot mail anyone. Both search before
  creating, so re-diagnosing an alert updates one ticket rather than opening
  several, and a rolling daily cap stops a loop mailing everybody.

Deployment
- Multi-stage Dockerfile builds the bundle and serves it from the API origin.
- docker-compose for local and single-host use; Gitea Actions runs the tests,
  builds the image and renders deploy/k8s with envsubst.

Two fixes found while testing: assigning a case returned a null assignee, and
add_event could leave an already-loaded history collection stale.

Known gap: the engine reaches OpenStack via `docker exec <region>-osc`, which
does not work in a pod without the CX-Tools containers alongside it.
docs/DEPLOYMENT.md sets out the three ways to close that.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/app/routers/linkage_router.py

@@ -0,0 +1,42 @@
+"""Linkage scan endpoints."""
+from __future__ import annotations
+
+import threading
+
+from fastapi import APIRouter, Depends, HTTPException, status
+from pydantic import BaseModel
+
+from triagelib import linkage as linkage_mod
+
+from ..auth import current_user
+from ..config import get_settings
+from ..models import User
+from ..routers.alerts_router import engine
+
+router = APIRouter(prefix="/api/linkage", tags=["linkage"])
+settings = get_settings()
+
+
+class EnrichBody(BaseModel):
+    region: str
+    openstack_id: str
+
+
+@router.get("")
+def scan_state(user: User = Depends(current_user)):
+    return engine.scan.to_json()
+
+
+@router.post("/scan")
+def start_scan(user: User = Depends(current_user)):
+    if not settings.feature_linkage_scan:
+        raise HTTPException(status.HTTP_403_FORBIDDEN, "The linkage scan is disabled on this instance.")
+    if engine.scan.state == "running":
+        return {"started": False, "reason": "already running"}
+    threading.Thread(target=engine.scan.run, args=(engine.snapshot.get(),), daemon=True).start()
+    return {"started": True}
+
+
+@router.post("/enrich")
+def enrich(body: EnrichBody, user: User = Depends(current_user)):
+    return linkage_mod.enrich(body.region, body.openstack_id)