Add Firefly III compose stack with k8s migration scaffold

PostgreSQL 16, Redis 7, Firefly III core + data importer, cron container.
Health checks on all services. .env.example documents all variables.
k8s/ directory scaffolded for future migration.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

k8s/README.md

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+# Kubernetes Manifests
+
+Placeholder for k8s migration. Planned structure:
+
+```
+k8s/
+├── namespace.yaml
+├── secrets/
+│   └── firefly-secrets.yaml      # APP_KEY, DB_PASSWORD, REDIS_PASSWORD
+├── configmaps/
+│   └── firefly-config.yaml       # APP_URL, DB_HOST, CACHE_DRIVER, etc.
+├── postgres/
+│   ├── pvc.yaml
+│   ├── deployment.yaml
+│   └── service.yaml
+├── redis/
+│   ├── pvc.yaml
+│   ├── deployment.yaml
+│   └── service.yaml
+├── firefly/
+│   ├── pvc.yaml                  # upload storage
+│   ├── deployment.yaml
+│   ├── service.yaml
+│   └── ingress.yaml
+└── importer/
+    ├── deployment.yaml
+    ├── service.yaml
+    └── ingress.yaml
+```
+
+## Migration notes
+
+- `docker-compose.yml` env vars split into Secrets (passwords, APP_KEY) and ConfigMaps (URLs, driver settings)
+- Named volumes → PersistentVolumeClaims
+- Health checks → liveness/readiness probes (already defined in compose)
+- The `firefly` bridge network → k8s Services with DNS names matching compose service names