feat: add feature branch deployments with per-branch isolation

Enable deploying feature branches into isolated environments on the same
k3s cluster. Each branch gets its own namespace (cam-<slug>), PostgreSQL
schema, and OpenSearch index prefix for data isolation while sharing the
underlying infrastructure.

- Make OpenSearch index prefix and DB schema configurable via env vars
  (defaults preserve existing behavior)
- Restructure deploy/ into Kustomize base + overlays (main/feature)
- Extend CI to build Docker images for all branches, not just main
- Add deploy-feature job with namespace creation, secret copying,
  Traefik Ingress routing (<slug>-api/ui.cameleer.siegeln.net)
- Add cleanup-branch job to remove namespace, PG schema, OS indices
  on branch deletion
- Install required tools (git, jq, curl) in CI deploy containers

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

deploy/overlays/feature/ingress.yaml

@@ -0,0 +1,26 @@
+apiVersion: networking.k8s.io/v1
+kind: Ingress
+metadata:
+  name: cameleer-branch-ingress
+spec:
+  rules:
+    - host: BRANCH_SLUG-api.cameleer.siegeln.net
+      http:
+        paths:
+          - path: /
+            pathType: Prefix
+            backend:
+              service:
+                name: cameleer3-server
+                port:
+                  number: 8081
+    - host: BRANCH_SLUG.cameleer.siegeln.net
+      http:
+        paths:
+          - path: /
+            pathType: Prefix
+            backend:
+              service:
+                name: cameleer3-ui
+                port:
+                  number: 80