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/init-job.yaml

@@ -0,0 +1,30 @@
+apiVersion: batch/v1
+kind: Job
+metadata:
+  name: init-schema
+spec:
+  template:
+    spec:
+      restartPolicy: Never
+      containers:
+        - name: init
+          image: postgres:16
+          command: ["sh", "-c"]
+          args:
+            - |
+              PGPASSWORD=$POSTGRES_PASSWORD psql \
+                -h postgres.cameleer.svc.cluster.local \
+                -U $POSTGRES_USER -d cameleer3 \
+                -c "CREATE SCHEMA IF NOT EXISTS BRANCH_SCHEMA"
+          env:
+            - name: POSTGRES_USER
+              valueFrom:
+                secretKeyRef:
+                  name: postgres-credentials
+                  key: POSTGRES_USER
+            - name: POSTGRES_PASSWORD
+              valueFrom:
+                secretKeyRef:
+                  name: postgres-credentials
+                  key: POSTGRES_PASSWORD
+  backoffLimit: 3