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>

frontend/vite.config.ts

@@ -0,0 +1,19 @@
+import { defineConfig } from "vite";
+import react from "@vitejs/plugin-react";
+
+// In development Vite serves the app and proxies the API to the Python backend,
+// so the two can be worked on independently. In production the built bundle is
+// copied into the backend image and served from the same origin.
+export default defineConfig({
+  plugins: [react()],
+  server: {
+    port: 5173,
+    proxy: {
+      "/api": {
+        target: process.env.VITE_API_TARGET ?? "http://localhost:8080",
+        changeOrigin: true,
+      },
+    },
+  },
+  build: { outDir: "dist", sourcemap: false },
+});