feat(renovate): add Gitea update automation

Run Renovate in Kubernetes to create reviewed image update PRs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

renovate.json

@@ -0,0 +1,45 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "config:recommended"
+  ],
+  "enabledManagers": [
+    "docker-compose",
+    "kubernetes"
+  ],
+  "kubernetes": {
+    "managerFilePatterns": [
+      "/k8s/.+\\.ya?ml$/"
+    ]
+  },
+  "packageRules": [
+    {
+      "description": "Keep private homelab images unchanged",
+      "matchDatasources": [
+        "docker"
+      ],
+      "matchPackageNames": [
+        "/gcr\\.forust\\.xyz\\/forust\\/.+/"
+      ],
+      "enabled": false
+    },
+    {
+      "description": "Require approval for major upgrades",
+      "matchUpdateTypes": [
+        "major"
+      ],
+      "dependencyDashboardApproval": true,
+      "automerge": false
+    },
+    {
+      "description": "Group container patch updates",
+      "matchDatasources": [
+        "docker"
+      ],
+      "matchUpdateTypes": [
+        "patch"
+      ],
+      "groupName": "container patch updates"
+    }
+  ]
+}