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/kustomization.yaml

@@ -0,0 +1,44 @@
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+namespace: BRANCH_NAMESPACE
+resources:
+  - ../../base
+  - ingress.yaml
+  - init-job.yaml
+images:
+  - name: gitea.siegeln.net/cameleer/cameleer3-server
+    newTag: BRANCH_SHA
+  - name: gitea.siegeln.net/cameleer/cameleer3-server-ui
+    newTag: BRANCH_SHA
+patches:
+  # Server Deployment: branch-specific schema, index prefix, UI origin, OIDC disabled
+  - patch: |
+      apiVersion: apps/v1
+      kind: Deployment
+      metadata:
+        name: cameleer3-server
+      spec:
+        template:
+          spec:
+            containers:
+              - name: server
+                env:
+                  - name: CAMELEER_DB_SCHEMA
+                    value: "BRANCH_SCHEMA"
+                  - name: CAMELEER_OPENSEARCH_INDEX_PREFIX
+                    value: "cam-BRANCH_SLUG-executions-"
+                  - name: CAMELEER_UI_ORIGIN
+                    value: "http://BRANCH_SLUG.cameleer.siegeln.net"
+                  - name: CAMELEER_OIDC_ENABLED
+                    value: "false"
+  # UI ConfigMap: branch-specific API URL
+  - target:
+      kind: ConfigMap
+      name: cameleer3-ui-config
+    patch: |
+      - op: replace
+        path: /data/config.js
+        value: |
+          window.__CAMELEER_CONFIG__ = {
+            apiBaseUrl: 'http://BRANCH_SLUG-api.cameleer.siegeln.net/api/v1',
+          };